I saw this product advertised at various places on the internet; it's a 2.5" HDD caddy to replace the optical drive in laptops. My question is, how to find out if the DVD drive has a SATA II or III interface? My laptop is a Dell Studio 1558.
SATA-II and SATA-III are compatible, and you are unlikely to see any difference in performance between the two on a laptop HDD.
So the basic answer would be: do not hesitate to buy this product, it will work for your laptop.
That laptop, according to a Google search, was released mid-2010, and I believe that the first Sata III equipped laptops came out in late 2010 - early 2011. I think Intel's Sandy Bridge chipset was the first to support it natively.
If you want to investigate for yourself, you can try use Speccy.
Sata II and III are compatible either way, similar (practically speaking) to the USB 2.0 and 3.0 ports