Well, zero is zero..... Above 30psi or so on the low side (there is only ONE pressure sensor, on the high side, so then the system is not running it reads the equalized pressure) the system should come on. If the compressor runs the low speed fan enable circuit is also on. High speed is engaged either from the thermoswitch or the ECU seeing high pressure on the refrigerant pressure sensor.
30A inline fuse for test leads is fine. I'd also test the leads with an ohmmeter first; quite low is normal (motors read very low when not turning) but a dead short is not. I have a bench power supply that is current-limited that I use for things like this; it will fold back and complain rather than blow something up if there's a short.
If refrigerant is leaking out then you have to find and fix the leak. I'd put a can of refrigerant in there with dye in it, run it a few minutes and then go look with a black light. Wherever its leaking will likely glow like a Christmas tree. Don't be surprised if its the condenser or evaporator, although the shaft seal on the compressor can leak as well -- or the service ports IF DISTURBED, and you have, are fairly common too. The hoses and O-rings, unless disturbed or the hoses are damaged, are quite a bit less-common. In any event the UV dyed refrigerant and a black light (or a refrigerant sniffer if you have one) will show you where it is. If the unit is at atmospheric pressure you must assume that the drier is toast. Beware not taking care of that; moisture + the refrigerant and oil makes an acid over time which will eat everything, including the evaporator and then you're ripping the dashboard out to get to it for replacement. NOT fun.
Note that the service ports are a bit tricky to replace only because its not obvious if you haven't changed them before. If they're not leaking leave them alone, but DO have a set in your toolbox -- they're cheap and if you have 'em and need 'em, well, you need 'em. The hex on the port and that on the pipe ARE DISTINCT but don't look that way if you're not careful, and you have to counterhold the pipe hex because its aluminum and will snap if you don't. The ports are NOT in tightly; the seal around the bore is made with an O-ring, not from being cranked down.
The drier must NOT be allowed to be exposed to the atmosphere until it is installed and then immediately evacuated. It will be ruined by atmospheric moisture in less than a half-hour if the ports are uncovered so it takes some finesse to change it as you have to be prepared to immediately pull vacuum (and being able to flush with moisture-free gas, usually nitrogen although a tank of scuba air works too since that's a -50F or better dew point, if you have the regulator for it, is a really good idea.)