You're not looking for spot-on numbers - they'll almost never happen. What you're looking for is acceptable ones. If your +5VDC is reading 5.18 volts, that's /quite/ acceptable. If it's only reading +3.4VDC, that is NOT.
And whereas you can check the voltage on the power supply as a first step, you should always measure your voltages on the board itself.
You'll always lose some in the harness and in the connections, that's to be expected. How MUCH you lose is a matter of course. If you read +5.2VDC on the power supply, and +5.1VDC on the board, that's acceptable loss. If you read +5.2VDC on the power supply and only 4.7VDC on the board, start looking for why there's that huge drop. Broken solder joints, oxidized connections, burnt headers - all reasons why that can happen.
The most valuable tool that you have in your disposal (besides your meter) is your eyes. Keep them open.