how can I tell if mechanical fuel pump has a bad diaphram without using pressure gauge?

 

Use one finger to plug off either the inlet or outlet side (just push the pad of your finger against the hole real hard), then move the pump rod up and down like it was working. If the diaphraghm is bad, you wont get suction or pressure. If it's not too bad, you may still get a little, but if you wait, the suction will slowly bleed off. It's bad.

 

if still on the car you can take the coil wire off completly and run a rubber hose from the carb side and make sure you go out of the engine bay and put hose in bottle with the help of another and turn key and see what kind of pressure you might have but please make sure its away from any spark and like rich says if it leaks its probably bad but if its going into the crank case its not good either and if off the car which is the safest way do like buck says