Can I use Photoshop to detect or find out with 100% certainty if an image is altered or not?
No, this is impossible. There is nothing intrinsically different about a doctored photo as opposed to an "unmanipulated" photo, so it is always possible to doctor a photo sufficiently well that it is indistinguishable from an original (besides such things as depicting impossible scenes). In fact, the photographing process itself involves manipulation of the image in various ways, and especially digital cameras that we have nowadays extensively alter photos on the fly as they take them.
When amateurs carelessly manipulate photos, they don't go to great lengths to hide their manipulation. There are certain common errors people make that make the manipulation obvious, this is how most "photo verification" works. However, those "rookie forger mistakes" are really not hard to avoid, even for rookie forgers (assuming they have a day or two to kill).
Detecting the forgery is a matter of spotting a mistake made by the forger, which will give it away. But the forger could simply not make any mistakes (or you could miss them), so just because you haven't found any mistakes doesn't mean it's not forged.
When an image is doctored or manipulated, is there metadata embedded in the altered image
Yes, for instance if you take a photo of the sky, then copy/paste a bunch of UFOs in it, Photoshop will add some metadata showing that the image has been altered in Photoshop. However, this is trivial to remove, so just because the EXIF says taken by camera so and so doesn't mean it's genuine.
On the other hand, if it does say it's altered by Photoshop, that doesn't mean anything either. A lot of people have cheap cameras with crappy firmware that can't get color balance right, or takes unnecessarily big photos (for instance many cameras will say they are 12 megapixels, but each pixel is so noisy that you are basically not losing anything by shrinking them to a quarter of the size). A lot of people just routinely open their photos in Photoshop, rebalance the colors, resize (Photoshop has much cleaner algorithms for resizing than many other software) and resave. So it doesn't necessarily mean they doctored it.
A more advanced general strategy is looking at statistical properties of the data. Digital sensors have intrinsic noise (eg. if you take a photo of a flat blue wall, some pixels will have a bit more or less red or green). The pattern of this noise should be uniform throughout a camera: For instance, if you take a sky photo with a very cheap, noisy camera, and then paste the photo of a spinning frisbee from a very high quality, low noise camera, somebody can look at the noise, see the dramatic drop around the "UFO", and conclude it's probably been doctored. Likewise certain characteristics, like sharpness of the image, will also be constant within a photo but vary between photos, especially if taken at different light levels or with different cameras (ie. if you are cutting and pasting from multiple different images).
Again the problem with looking at noise (or light, color, etc.) statistics is that it's very easy for me to artificially add noise that looks "about right", then print the photo from a very high-quality printer, and scan it back with a very cheap scanner (or even photograph it with a camera), then doctor the EXIF and claim it is the original. The editing will be virtually undetectable. This is maybe half a day of work for me, and I wouldn't do it for something like a quick forum thread where nobody will spend more than 5 seconds on the image anyway, but if I wanted to be on the news with a UFO story it's not that much effort.
Error Level Analysis is a similar technique, but has to do with noise introduced when compression like JPEG is applied. Most cameras apply JPEG immediately, and the JPEG adds artifacts to the picture. When you then alter parts of the photo, the artifacts in the altered region will be different. But the problem is, once again it's not very hard to just configure a camera to not use JPEG and take uncompressed pictures, and only compress after you've done all your photoshopping, then ELA will be useless.
Then there are the old-school methods like looking at aberrant lighting, shadows, and so on. Obviously there's nothing stopping a forger from just getting these details right, and with many UFO photos there is so little useful lighting/shadows that you can't do this anyway.
Note that it is also possible to physically construct a fake UFO scene (there have been many notorious examples, like the "pot lid dangling on a string" or what has turned out to be cloud formations) and then take a picture of a it - in which case the photo is of course not doctored in any way (so there is nothing to detect) but that doesn't mean it's a genuine UFO photo.
Or is there some other tool or method that can assist?
Your best bet, if you really wanted to know, is probably to find a professional image analyst who has been doing this for many years, and ask/pay them. Of course you have the problem of figuring out who will do a proper analysis and who will just take your money and lie to you - it's not like there's a perfect litmus test anyway, any analyst will at best give you a probability that the image is altered, and so if you pay someone to the analysis there is little consequence for them if they are wrong.
With forensic image analysis, no two cases are the same, and a lot of it comes down to intuition and skill of the analyst. The skill is something that cannot be gained without extensive experience - learning to do it yourself would probably take you years, and isn't worth your time unless you plan to do it for a living.
I was reading some news about fake UFO photos and this question came to my mind.
With UFO photos in particular, I'd say you are better off just forgetting about the analysis, and simply assuming it's fake. There have been millions of UFO photos, and not a single genuine one. At this point, even if there was genuine documented extraterrestrial activity, it would be impossible to believe photos alone - the well has been poisoned (by frauds and hoaxes) to such an extent that photos are simply useless as "proof".
In US criminal court, the standard of certainty is that guilt must be proven "beyond reasonable doubt". This is a fair standard to go by, and if you apply it to UFOs, proof of photo authenticity beyond reasonable doubt is impossible, just due to the issues I bring up in my answer (there are also many issues I haven't brought up).
In science, the standard is often that there must be at least 95% chance that the null hypothesis (eg. that the photo is genuine) is not true. If you apply this standard, then it is likewise impossible for any genuine UFO photo to meet it outside a few specific circumstances. And even then you can only claim a photo meets the standard in a very naive way - you can at best say "it's very unlikely that this photo has been manipulated after being converted to JPEG for the last time" - which of course doesn't say anything about what happened before the conversion.
These are the two major standards of proof that are commonly used. They are actually quite lax, since both are meant for credible, realistic claims. Genuine photos of UFOs are an extraordinary claim, and should be backed by extraordinarily strong evidence - with a photo alone, this is impossible.