Well, usually anyone who knows their horse and has general horse sense can tell. Basically, he's just going to look pissed off.
My last horse had several issues and the right bit was critical, so when I started leasing him, I decided to try different bits because the bit the barn had for him was not working too well. I thought maybe he might like a D ring with copper rollers, to give him something to play with. He pinned his ears, traveled sideways, and stuck his nose in the air. He was sending me a clear message "Sorry, I hate this one, can we try a different one?!"
I tried him with a loose ring snaffle and he was happier than I'd ever seen him. And not every horse will like the same bit...you just test until you find one that your horse seems to be calm and happy to have in his mouth.
Cinnamon Whiskey 11 y/o 15hh Chestnut AQHA mare, 2'6 Jumpers