When I play doubles and am the non-receiving partner, I stand on the service line and face the other half of the court. As the server starts serving, I manage to watch him/her impact the ball, and also quickly see the ball bounce. I don't think it would be feasible in a pro match, but I find that I need both views to accurately judge whether the ball is in or out. The overall trajectory, plus what happens near the service line. I am then usually completely confident about the call and will try to overrule my partner if he/she contradicts me.
I am thinking now if just focusing on the service line will work.