It depends on what standard they are. The standard of musicians I'm looking for would be able to play music they'd never heard before from charts, and at least jam to a good standard over changes.
Then it's about whether the bass and drums lock. The singer and guitar can think whatever they want. If bass and drums doesn't work - it won't work.
Finally is there a relaxed atmosphere while you're playing and chatting?
Oh, and how many fights has the singer had...