Whatever method you choose - just make sure you can hear the "live house" too. Otherwise you'll never know if your volume is appropriate. In smaller situations, I make sure I can hear the people talking that are closest to me, and I watch to see if they need to "lean in" and strain to hear each other. One thing I have always done is this:
I make the first thing anyone says to me.....
"Could you turn it UP, we want to hear you!"
THat way the night begins on a positive note, and not a critique.
I usually use the keyboard speakers as a monitor (even vocals) but for larger venues, or odd setups - I have a Powered Core Sysyem (hot spot)stand mounted monitor. It's very loud and doubles as a small PA for a piano bar gig, or to use at coctail hour with a MD or something like that.
I can't sing the praises of good monitors enough. I have NO hearing loss and I intend to keep it that way. Make sure you can hear what you need to - it's vital for a good, solid performance.