I've learned that expecting people to know what I want them to say usually results in my disappointment, because as much as I want them to be, most people are not mind readers.
I tend to just let things play out naturally, so I can't imagine myself asking for a commitment of some kind, especially before even going on a date. But if I was concerned that someone might not be ok with poly, I would probably just ask them what they think about being in that kind of situation. I don't think it's presumptuous to discuss relationship arrangements before a first date, given that you're chatting on a dating site and those kinds of questions are to be expected.
I guess my situation is also quite different from yours. I'm married and have a girlfriend, so no one's likely to mistake me as being available for an exclusive relationship.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker