I can never tell whether those groups are playing live music and singing live or not. Not unless there's some big mistake, like pretending to sing into the mic when the music has already stopped or something.
1. Do their rigs have power and are they actually turned on. Those are two big giveaways, especially when it comes to electronic music. If not, you know they're playing air keys.
2. Note how the singer is holding the microphone and whether you hear an acoustical change reflecting that. If not, they're likely miming.
Sometimes it is tricky and you have to go by the bands personalities and past performances to suss it out.