Use your own ears I always say....
But that only works if you know what to listen for and sadly many players don't have the stage experience to realize that. A few times I have watched and listened to acts on stage in different venues I have performed with who were playing in different rooms ........during breaks I would chat a bit with them, talk shop, etc, ......to my surprise after noticing one players PA sounded muddled and way too Bassy, drowning out the vocals big time leaving them with no clarity what so ever,.....when I brought this up to him he merely shrugged his shoulders and says "Sounds good to me"
And this has happened more then once,......I have played on many big stage venues, arenas, etc, thru the years with some Headline acts years ago, with thousands of people in the audience using different professional Audio technicians out front whom I learned many tricks of the trade to properly EQ the room sound during a show & how to "sit in the mix" also which is very important to not overdo the other players in the band. Now as a OMB I use many of those things to make My Sound the best it can be. And also your instruments must be properly EQ'd as well,...You may think you have the best PA in the world but if it isn't correctly setup, & EQ'd properly it's all for nothing. Sounding good on stage is Science, Talent, & Lots of Stage Experience, but when that all comes together it's Pure "Magic"