I go by location, cadence, repetition, situation, etc. Some you hear and just know… nothing can imitate that volume/hollowness. If it sounds like Godzilla at the top of the mountain... It’s no hunter. But… I have heard some downright terrible bugles from nice bulls.
If you’re getting bugle responses but they are moving away — bull. If they quickly answer a cow call— bull. If the bugle comes from far off trail or in a deep draw — bull. If a convincingly sounding bugler starts getting quiet at the end and trying to cut around downwind — assume bull. Repetitive calls, calls from a great glassing high point, from where I know there is a trail, etc. I will typically assume itss a hunter. But even then you never know with 100% confidence unless they do something “fishy”. If I’m ever in a bugling exchange with something I believe is a hunter, I’ll typically intentionally throw two exact bugle, 3 chuckle combos. In succession. More than once the “elk” magically stopped calling.