It's simple, you can't, and yes it sucks. Even if you buy from a hatchery and the chicks have been sexed there is still a small chance you'll end up with a roo. I have this same fear and I've been worrying every day for the past 3 wks. If I could do over I'd buy two extra chicks to start with and after it was certain who was roo (if any) then I'd happily get rid of them and still have my ideal flock size, I'd rehome my two least favorite hens as well. I have six chicks and I love it.
Katie
wanted to add that I got mine from a farm store that was only selling pullets of the breeds I got, so mine were all sexed by the hatchery.