The best way is to purchase the FSS report for the state he is in. The FSS report gives you the potential for all ratings.
If you can't afford FSS for the state, you can do scouting reports, which will give you the potential for 3-5 ratings each.
WE has nothing to do with potential in HD. Each rating has a hard cap (potential), the WE only affects how quickly they reach that cap (along with practice minutes and playing time).