Forrrest wrote:... is there an easy way of checking that the two circuits we're given actually are on different 15amp breakers? (instead of just trusting the soundman)
Ideally you want the lighting on a completely different circuit from the PA and your stage gear, because not only can it draw a lot of current, but depending on certain factors it can add noise to audio. It's also nice to be on separate circuit from the PA to avoid tripped breakers because their amps can draw a lot. They should be giving you guys at least 2 separate circuits for yourselves (and then lighting one and PA on another) - I'm surprised they would link several receptacles on the same circuit at a venue. If when your amps went dead, the lights went out and/or the PA went dead, then you're sharing a circuit with whatever else went dead - only the overloaded circuit would trip, not several.
There are three reasons breakers trip:
Overloaded (too much current) - breaker trips so the circuit doesn't overheat.
Short Circuit - A hot is making contact with (or arcing to) the neutral.
Ground Fault - Power going to ground - similar to short circuit. Depending on the building and how much power is going to ground, this can trip the mains and shut the whole building down (I've seen it).
But I think it is most likely that you're overloading a circuit that's shared.
To answer your question about checking what receptacles are on what circuit: Use a multimeter, or better yet for $20, you can buy a sharpie-sized voltage detector (aka ticker). Tickers are handy - they've saved me from getting hit by 120 V and even 347 V (never take anyone's word that a circuit is dead, unless you check it yourself). Find the breaker for the stage, turn it off and check the receptacles with the ticker by just sticking it in the hot plug or use a meter across the hot and neutral for voltage (make sure it is set to read something proper like voltage, and not continuity or something - the ticker is a more foolproof method). A ticker will flash lights and beep like crazy if that receptacle has power, or nothing at all if it's dead. If it shows dead, then you know that's the right circuit turned off, and then go around and check the other receptacles on the stage and where the lights and PA plug in to see if they went dead with that same breaker off. If they did, then they are on the same circuit and sharing a breaker. Figure out the other circuits by turning a different breaker off, and seeing what receptacles are affected. This seems like quite a bit of time, but if you play the venue a lot, it may be a good investment to understand the circuit limitations of the venue.
Someone at the venue should be able to help you with what breaker(s) to start checking so you don't have to go through them all. Or it should be labelled in the panel. You can also short the circuit and see which one tripped, if you're in a rush and can't find the circuit because you're not allowed to randomly turn on and off circuits till you find the right one or there are several panels all over. But this is a last resort and you need to be extremely cautious, because you're basically sticking in a piece of insulated wire and connecting the hot and neutral, and it can arc and leave char marks on the receptacle - or worse, if the building has crappy breakers (ahem Federal Pioneer *cough*), they may not even trip when they are supposed to. I was working on a live lighting circuit with someone once and a wire got its insulation skinned and grounded out - BOOM! A big arc, smoke, and even a tiny bit of molten copper spray on him - we got down, and when we checked the panel it wasn't even tripped, all that power was still going to ground and the breaker didn't read a ground fault. So be careful not all buildings, electricians, or even inspectors live up to the same standards. But a ticker test after a breaker is turned off is very foolproof, quick, safe, and you don't need any metal on metal contact.