You could ask your partner before-hand in a general chit-chat about cum, or, as you’re about to shoot your load.
If you’re on the receiving end and you don’t want to swallow his cum – but you’d like to feel it in or around your mouth, one tip would be to open wide and put your tongue out. This gives him a soft landing area to aim for, and you can avoid it potentially getting deeper inside your mouth, or at the back of your throat (which can sting a bit and maybe even give you a croaky voice for half an hour if you end up swallowing some). It’s intimate, and potentially a way of telling someone you’re really into them. Dramatically spitting out a guy’s cum combined with the retching sound you make when you taste something bad can dent self-esteem and make things awkward.