From what you've done so far, there is no way to eliminate the cpu or mobo. If your stick of ram works in another comp, then I would say its prob fine. Now you have to do the same with the cpu and mobo, until you identify the bad part. Another thought, are you sure the mem is compatible with your mobo, ie some mb's don't accept ECC RAM, or once i tried puting tiny BGA sdram in my mobo and it didn't work, even though it worked in my GF's comp.
I've run into this many times building comps, or having a part die. You just have to work through your parts systematically. Its not too bad when you have spare parts handy, but if you don't well...
Once, I suspected i had a dead slot 1 celly, but had no other comp to test it in, so i actually sneaked into the research lab i was working in (when I was in college,) on a Sat. and tested the CPU in a computer there I knew to be a slot 1. Turned out it was bad, and I sent it back for a replacement. (hee hee, don't try this at home, was pretty dumb!😀)
If you can't find ways to test, you may just have to send parts back for replacement, or you may be able to take them to a comp repair shop to be tested. Good luck.