The refer you should ask them to turn it on using electric the night before. You can tell it's running when you get there by putting your hand on the chimney...if it's hot, it's operating, if it's cold, something wrong.
Now put your hand in the freezer, should be very cool after running overnight. If it smells like ammonia in there, or if there is any yellow residue outside under the vent cover on the refer coils (really look waaaaayyyy up there as best you can - take a flashlight with you), it's a bad refer...no good, subtract $1,000 from your offer on the rig.
If it's a multi-function refer, and it has gotten cold, you can then switch the refer to gas, open the refer door, and let it warm up. The gas flame should start and run. Make sure it runs for more than 10 minutes because sometimes they shut off to soon due to mis-adjusted or bad thermocouple. (Simple inexpensive fix).
If it's a 3-way; 120 volt, gas, and 12 volt, nobody uses the 12 volt heater element because it's power hungry so don't bother with it. If the refer runs on 120 volt and gas, you're good. (Except for residential refers in rigs, they all use 12 volt for the controls too).
With the water tank, just ask them if it's 120 volt and gas. If both, when you get there, have them turn on the 120 volt element. 10 minutes later, just touch the tank or turn on a faucet. Should be warm. Then have them turn on the gas, let it run. Than test the water for hot.
