From what I remember battling greenhouse whiteflies back in da day...
Yellow sticky traps! They are the same concept as fly paper rolls but they are meant to attract common greenhouse pests, whiteflies being the easiest to trap with those things. You can find them at any well stocked garden center. Safers makes some, and I'm sure there are plenty of others. But it's like the EASIEST thing to use and it will also tell you if you have other pests... when you see specks on it, look close... you might find other bugs you didn't know you had!
Good news, from what I remember they aren't as destructive as thrips or mites.