Let's be honest here, we all know a talented player when we see one, including our own.
I think what is important is not what we always see, but what others see and how our players see themselves. It is our responsibility to provide opportunities for them to compare themselves against others (not just on the HS field) and to provide opportunities to improve.
I am strongly against a parent telling their player they are better than others, just as I am strongly against a parent jumping all over their player because they don't meet their (parents) expectations. There are some things our kids have to figure out in life and where they belong and where they want to be in the scheme of things is up to them. The only time we got on our son a** was if he was lazy. We offered support if had a bad outing. We never said you are better than that, we always said you could do better than that. We never compared him in conversation with him to anyone but himself.
I agree with some points that IFD and CD have made, especially consistancy. If you faced a top player one time and he beat you (either being a pitcher or hitter), wa he that good or were you having a bad day and vice versa? You need to face that top player quite a few times, with metal and with wood, maybe for 2-3 years before you can determine if you have it or you don't, that's a true test, IMO. My son used a few players he faced as his benchmark. One was Matt Weiters. On occassion he struck him out, on occassion he blooped him out, on occasion he got on base and also got jacked. But each time it was a battle and there were quite a few of them. He knew he was most likely facing the best hitter in college baseball and did well. He learned a lot from it all.
Seeing your son in the lineup consistantly through his career is also a good measure as to his ability, especially if you face tough competition.
JMO.