I believe the question was about testing compressors and not test cords. Now to test a compressor fully you should not only take in to account the electric motor but its mechanical components as well.Starting with the electric troubleshooting.A compressor ( and I mean the motor) can fail and have either open windings or shorted windings, note that the windings can be shorted among themselves or to ground, chassis or the case (it is all the same thing). Open winding means the compressor wont run and probably trip the overload protection, a shorted winding will either activate the overload or blow a fuse (depending how solid is the short)Now an open winding is always conclusive test but to distinctively conclude that a winding is not shorted is difficult because motor windings have a very low resistance. You will require a good deal of judgment (experience) and a very precise instrument to test compressors in this fashion. One exception to above is a winding shorted to ground, this is always conclusive when it shows up, however even these failure modes are not detectable all the times and for this reason many professionals use insulation testers to uncover them. At this point it is a good idea to use a test cord and see if the compressor operates. Note that with use and age compressors can require more starting torque to turn and may require a starting capacitor or hard start to get it rolling.If your compressor still refuses to run the problem is in the overload or mechanical in nature, if you believe the problem to be mechanical let us know.