Front quarters larger than hind, neck as broad as torso, and waist as big around as chest.
There's an old metric that if you shoved a 2×4 through behind his front legs and lifted, would he tilt forward or backward? If backward then he's 3.5 or younger. If he balances he's 4.5. If he tilts forward he's mature. While not exact it's pretty darn close.