Computer (switching) power supply -- assessing quality/problems "live"

[13hm13]

PSU issues, especially in noisy environments as standard desktop computer or server, can be tricky to diagnose.

Say I've got a generic PC computer and suspect PS issues, but I don't want to:

(1) dissemble the PC
(2) take it down (power it off)

What symptoms indicate main PS issues (and not, for example, motherboard capacitor issues)?
Can I "probe" or oscilloscope various power connectors ... look for "tell-tale" ripples or noise coming from the main SMPSU?