Depends on the venue, the singers, the instruments... Whole bunch of stuff. Basically you get familiar with the tools and trust your ears on when to use how much of them.
For reverb, less is definitely more. Usually in a live setting no reverb is required because the room provides its own reverb. (That's what reverb is for, to simulate the sound in a virtual "room" setting.) If you do use reverb make sure everything in the mix has the same kind of reverb.
See, the auditory center of a person's brain uses the reverb sound to reconstruct a mental image of the room the sound is in. If different instruments and vocals use different kinds of reverb the brain gets multiple room reconstructions and the person gets a generally bad impression of the whole mix. Best to set one kind of reverb and just control how much it is applied to separate channels in the mix.
And if you can listen to it and tell it has reverb, it is too much. Best reverb is when the whole mix sounds better, but you can't distinctly hear the reverb on it.