Rob, it is evident just from the appearance, and quick distinguishing between a hard, semi hard and soft bull comes with experience, so imperative to trust your experienced PH. Unless I misunderstood, I don't agree with what LvW said regarding the height of the tips. Sure, the tips of a Buff will drop with age, but it is not the sole distinguishing factor to age a Buff. The height of the tips has more to do with genes. Some Buffalo will never drop to bellow their bosses, no matter how old they are. I speak under correction, but I believe the Addo bloodline is known for that.
A Buffalo will also harden from the outside, back, towards the inside front. So, it is possible for a Buffalo bull to look hard from the back, but still soft in the front.
It always makes me more comfortable to see a Buff from the front, to be entirely sure, before taking the shot. If one does not have that luxury, and the Buff looks hard, at least from the side, the next thing you should be looking for is the Roman nose, which is only evident in older bulls, as well as the small dewlap that develops on these aged bulls.