I've done this before... my hint is
1.. don't overrule unless the players ask...
2.. just look at their reactions /both the hitter and the line caller..
in the past few years that I have been watching JR. tournament I see a lot line call questioning ,especially from some top players.. sometimes they all think that their hits should all be in... I haven't figures out yet if they are doing this to slow down the play/gamesmanship or they are just freeking idiots..
...the other thing I've been noticing is that some player who really ''don't cheat'' just make bad calls ... most of those bad calls come from the fact that they stop paying attention to the ball 'in flight'.. they assume that the balls will sail out and take their eyes off the ball and just call it out..
one thing I can almost assure you.. girl tend to cheat more than boys ,, just because they are better actors..
I hope you can watch for one thing that is really bothering me .. the trend of coaching.. parents/coach are by the fence coaching their kid all the time and really getting into other parent's face outside while they are watching .. a real sneaky thing they do is talk or speak in a different language .. I would love to see more official take a bit of control of that stuff.. just walk up to the talker and tell them that if you have something to say USTA rule states that it "" must be said in English "" ..