TOPIC: What is the best way to check an ac unit?

Does anyone have a tried and true way of testing ac units?  Do you take temperature dif checks if so where and with what type of device?
Do you just check at the nearest register?  If so what temp is ideal? 
Thank you!
Have a great day all!

Mike, I always check the temp at the return and the temp at the register. I'm looking for about 16-20 degrees difference. I think that's pretty close. I'm not an HVAC guy, I think someone said David Anderson was very knowledgeable on the subject.

Michael,
What type of thermometer do you use. 
I tried checking the td between the suction lines but the readings can vary so much from one minute to the next .  I'm thinking maybe my inferred thermometer is reflecting off the surface and giving me bogus readings because the readings aren't always consistent.
Thank you

I really hate to be redundant over the years; but the air-conditioning equipment does not even speak the same language as the thermostat!.

It doesn't matter what type of thermometer you use,  if it measures dry bulb temperature.

The air-conditioning equipment can only understand wet bulb temperature or "latent heat"!

The thermostat and the HVAC equipment can not even speak the same language, but the home inspector is expected differentiate between the two.

The short of the long is that the HVAC equipment removes latent heat which subsequently lowers the indoor air "sensible" air temperature to the set point of the thermostat.

Unless you use psychrometric evaluation technics described by ashtray (American Society of heating and cooling refrigeration engineers), which is so far beyond home inspection evaluation it's not funny, you can't even fathom a potential recommendation if you were even inclined to think there was a problem.

HVAC equipment can operate 100% efficiently and the indoor air conditions can not be met!

Why, because the equipment is not properly designed for the application.

I have a cheap infrared thermometer with a laser pointer I sometimes use. For a more accurate reading, I have a Fluke Multimeter with thermometer reader and probe attachment. Usually I just use the cheap one at the registers and returns, but it becomes less accurate the farther away I shoot the light beam. As for the outside, I don't take readings there. I just know that the biggest tube(that has the insulation) should be really cold, and the smaller tube will be ambient or hotter while the a/c is in operation. Sorry, I know it's not very scientific, and the HVAC guys are probably laughing their asses off at my inspection procedures, but that' what I do. The difference between the return and registers should be at least 15-20 degrees after 15 minutes. I just report on if it's less than 15, I don't know if there's a max at 20, because a gas furnace can put out 110 at the registers with a 70 degree input at the return. Maybe the rule is for electric heat pumps only. And if it is less, I still am careful not to call the appliance defective, I just note what the diff is and what it should be.

I knew you could answer this one better than me, David! Now does that mean that if I measure less than 15 degree diff, then I should be careful calling it a defect? I usually don't attempt to explain why, and don't pretend to be qualified to know why, I just report the difference should be looked into by a professional HVAC Contractor.