Question for dressagers (or anyone who has an answer)

My friend is having trouble telling when her horse is stepping up properly as he has a tendency to trail his hocks but can do it while seeming nice and round and relaxed. Maybe TOO relaxed? Her 'trainer time' has been cut recently and she's having to work on her own more but wants to be sure that she is working correctly and says she can't tell when he's correct. Do you have any hints of how one can tell from the saddle if the horse is actually working? He's very good at faking the work but will do it properly when it is insisted upon.