These are interesting posts. I don't do refg., just res / light commercial HVAC. Truck is loaded with parts mostly, so don't have a lot of room. 2 large nitrogens, gauges, scale, leak detectors, digi-clamp on, thermometers, etc.
Best tool I have is my ears. Listen to the customer for 5 minutes and ask them questions. Even if the customer doesn't know what's wrong, you should be able to narrow down the problem. Start out with basic dumb questions, like does it do it all the time, when was the last time it worked right, does everyone else in the building think it isn't working right, etc. Listen and process the info, and then ask more specific questions. You should have a good idea where to start looking before you ever pull out a tool.