Very true Gameboy, good point. Yet, I think most of us realize that. Due to how difficult it can be, in my opinion, the best approach is that if you have ANY QUESTION, you give the call to your opponent if you can't get to the ball.
If it's during a point and you are returning the shot, just don't make the call and keep playing the point. Always give your opponent the benefit of that doubt.
Yet, if you do clearly see a ball landing out, even if it's close, by all means, make the call. Finally, if you are playing a USTA Tournament for example and it's a real problem (a pattern by your opponent), don't argue too much, or get too upset, just get a linesperson from the desk. This overall approach worked for me, even from a very young age out on the courts.