If you keep the wings level and the nose pointing at one point on the horizon with no yaw and the wings perfectly level and the nose hasn't moved off that reference point and the wings are still perfectly level, and the reference point is still where you left it, that is in balance.
Absolutely all of this exercise can be done looking outside, with a brief check inside about every five or ten seconds to look at speed/power combination, balance and height, then back outside again. But don't move your head too much until you have the reference point burning an afterimage on the windscreen (with the appropriate lookout for traffic etc.)