As has been pointed out in the question you linked to, one of the diagnostic steps is to boot the computer without any RAM whatsoever. Provided you have a system speaker connected, you should hear continual beeping in some regular pattern. This will indicate the board is working to the degree that it detects that there is no RAM present. If your motherboard is good, there is a system speaker, the processor is good, the processor is sitting in the socket properly with no bent pins, the heat sink is properly mounted over the processor, and the power supply is good, then you would hear the diagnostic beeps. Notice, I said nothing about requiring a hard drive, or optical drive, or video card, or even having it connected to a monitor. You are simply connecting the basics, and attempting to force the motherboard to behave in a predictable manner.
If you do not hear any beeps... then one of those things is bad/missing. It might be a bad motherboard. It might be a bad processor. It might be a bad power supply. It might be as simple as the heat sink not sitting on the processor properly.
Why would you attempt to force the motherboard to tell you that there is no RAM present? Because you need so little in the way of components to test that, and it presents you with a very short list of things that might be bad. Also, if you get the diagnostic beeps you expect, then you can progress forward from there.
So, that's the first thing you should do.
How do you test your power supply? Without a PSU tester, you would need another computer to use it in, or another power supply to use in place of it, or a multimeter and a small wiring diagram showing you what wires should produce what voltage (and what two connections to short to make the PSU power on. You CANNOT assume that since the fan kicks on, that the PSU is working the way it should.
How do you test your processor? That's a little more specific. You would need another motherboard/computer that you KNOW works to put the chip in. That... or an expensive processor tester that not many people (or even repair shops) have.
How do you test the motherboard? Again... that's pretty specific. You would need another processor that is made for the same socket.
So... out of all those things, and how you would test whether or not they are working properly... testing for the presence of RAM is quite easy to do.