Warmups can be hard to tell. I look for bad technique, wherever it is, because that is the shot that will break down under pressure. Like a wristy groundstroke that requires lots of small muscles. They'll not be controllable under pressure. I played a guy once who had a long, slow takeback on his forehand. So I hit it harder than usual to his forehand on big points and sure enough, under pressure he slowed down and couldn't get his racket back in time. And then he hit the ball wide.
Sometimes "bad" technique can be very steady though, and that is sometimes not a good thing though it's usually not a weapon.