Not sure what level you play at but for USTA league play.
My 2 things I look for
1. I look for fitness + age
2. which side they spend warming up more (typically their favorite stroke and the one to avoid)
Biggest thing for real...league tennis has some people that have been playing in it forever it seems. I have a few team mates that have been swinging a stick with USTA since 1996 and these guys have pretty much played everyone or a0 tleast seen them play...unless it's a new kid...so often I just ask their advice.
My captain is very good about this in fact while we are being told the line up and what courts we are on he always throws in a few tips before sending us off.
sometimes you can see the instant reactions when right out the gate you hit 2 to 3 different shots to their weaknesses and instead of trying to figure yours out they fall into a defensive shell and at that point that match is yours.