If I power a motherboard without CPU/RAM, what will happen?

Booting a motherboard without RAM should cause beep codes.

However, from the few motherboards Ive powered that had no processors, no beep codes were generated.

I'm going to answer this as it happened for me:

• The 2-digit 7-seg display to have some diagnostic code, probably something along the lines of "No CPU"

For this specific motherboard, sigh, no such luck.

• Power LEDs on the board itself (if they exist) to light?

Again, not that I remember.

• Whirs or full-action from case fans or PSU fans?

Yes. The case fans did indeed spin up, and the DVD drive started attempting to spin up the nonexistent disc in it. (and appeared to proceed to put itself in a reset loop, presumably because it couldn't find anyone to talk to.)

One more thing: the purpose of this was to find out if my PSU or my motherboard was at fault, as nothing powered up when connected normally.

Despite getting signs of life from the non-mobo case components attached to the PSU, the PSU was at fault.

The fact that I got signs of life when I shorted the PSU's motherboard connection lead me to believe that the motherboard was at fault. (When the whole thing was connected, nothing powered up.) In my case, this was a mistake.

If you're debugging hardware, I highly recommend seeing a component work with a known-good setup in a full configuration before declaring it good; in my case, I didn't narrow in on the PSU until I swapped it out for a known good PSU, and everything else started functioning perfectly.