Great question that I'm sure we all wonder. If anyone of us actually could accurately predict that child has 'IT', I think IMG would would pay us handsomely!
Here's my thoughts:
First, what level of 'IT' are we talking about? A state champ? top D1? ATP top 100?
At age 7 or 8, even a perfect definition of 'IT' really can't mean anything more than a strong sectional player - way too many variables ahead. I recently tried to see where one of Macci's recent 'sure things' was at and it looked like she's playing golf?? And what about that kid that was on cover usa today and went to france?? (silva I think)
As you hit 10 you can maybe get excited to graduate into the 12-14 'IT' group - of which a very small number will be say top 50ish D1 players.
So now, if we're talking ATP top 100?!? Not only does the kid have to have IT, but a hundred other things need to fall into place, much of which you may have little control over. That includes coaching, mental and emotional stability, toughness/injuries, finances, patience/persistence.
As to what IT is, I definitely would not start with ball striking. I'd start with heart, persistence, determination, and movement/athleticism. I see hundreds of local and sectional kids with great ball striking but missing basic determination/mentality/atheltism to do anything with it.
Running through the proverbial fence/wall as reference above is certainly a prerequiste and dragging them OFF the court (unquenchable passion) is also a very good sign in my opinion.
At the end of the day, a really good athlete with incredible mental fortitude and persistence and passion is who I'd put some chips on. Next I'd look for anticipation and decision making/court sense. Then hands and techinque and ballstriking are the next level items I'd say.
Just my 2 cents...