I'm not buyin' into this one, its bloody hard. But when you see a really big fellow, you know!! Its helpfull to compare the apparent horn length against the length of the ear, this at least allows you to discount a smaller animal that may have appeared larger.

We talked about the big bulls running through the paperbarks with their head held sideways on another thread.
When the horns of a grazing animal reach up over its shoulders, I'm already stalking!