Not every plane will knife edge perfectly. It's been a while since I flew mine, but I remember it did couple a little bit. To tell if your balance is off in KE, if it flys to the belly, it's tail heavy. Flys to canopy, it's nose heavy. The reason being, in those balance situations, your elevator is compensating for the balance. A nose heavy plane will dive by nature, and a tail heavy plane will climb. So to make it fly level in normal flight in those situations, you compensate with elevator trim. When you go to ke, you will see the result of it.

Really the best way to tell if you're balanced ok is how AeroDave suggested. Roll inverted and see how it goes. Also, if you jump over to the pro bro site and search for balancing a profile, there's a really good write up on all the steps to balance a profile out for all points. Hope this helps!