When/How do you know if your PSU is hindering your OC?

[lopri]

Please read the title.

Except an obvious case where voltage rails fluctuates like there is no tomorrow.

 

[Avalon]

If you put in a new component (stronger PSU, extra HDD, beefier video card), and your rails are lower. Or if your rails are noticeably lower when OC'd as opposed to stock. Probably a bunch of other things.