The best way to know what a 50"+ bull looks like on the hoof is to view a lot of moose in normal gait. I could post pics of bulls ranging all around 50-60" (some smaller than 50"), but judging a bull on the hoof standing 8-9' weighing close to 1700-lbs is NOT the same as trying to imprint a "size memory" of a bull on the ground. So i wont waste your time with pictures, but i do have hundreds in my Float Draggin' book and videos
So, that said, here's some rules of thumb I use:
1. Broadside view with bull's head 90 degrees to your position: If a bull's antler tips extend to the rear of the shoulder hump, he's wider than 50"
2. Head on view with rack wider than a bull's body. If his antlers curve UPward vs flat-lying spreads, he's likely under 50".
3. Rule of 9s or 10s: If you have a head on view and clearly can see to measure the width between its eye orbits, a good safe bet is to expect this distance to be roughly 9" (textbook theory is 10"). I use the 9" rule to have a more conservative estimate on overall width. Once you have a 9" rule between the eyes, "judge" the 9" segments of antler width right and left of the eye orbits...giving you an estimate of total width. I use this only as a source for dual confirmation, say, with the broadside view with antler tips in proximity of the shoulder hump.
4. Browtine count: straightforward though difficult in many scenarios. Dual confirmation required for safe judging.
When all else fails and you still dont feel confident...leave that bull for me to eat....