If your talking about a PCB that someone else has done:
Does it match the schematic?
Does it conform to EMC, UL, standards etc?
Is the creepage & clearance correct for the voltages?
Are all the components SMT - if not why not?
If they are not SMT then can they be easily assembled - if not why not?
Does it meet the spec?
Is it 100% routed.
Is it error free?
Is the parts list correct?
Does it have an assembly drawing?
And so on.
A lot of it is stuff that was forgotten the last time - stuff that has caused issues in the past.
It comes with experience.
Oh - not to forget - is there someone else to blame when it all goes wrong....