I'm sure that most of us who watch tennis on TV will have seen the incredibly fine margins with which Hawk-Eye will call a ball in or out and it's clear that if the ball touches the line, it's in.
So how do you approach making line calls when you're trying to play at the same time? For me, the only thing to do is play everything that might be in [when I'm practising, I play everything I can reach because every ball you hit is another opportunity to learn] and if you're unsure, it's in.
Firstly, it's the fair and sporting thing to do. No-one is perfect and if you're calling in-balls out, not only are you cheating your opponent, you're cheating yourself. Secondly, you will become a better player by taking balls that might be a little wide or long.
You can only be accountable for your own behaviour and not that of others.