My oldest lab, Yeller, invert sneezes ALOT. For her, it's mucous in the back of the throat that can trigger it, usually after she's had a big drink.
My little lab has some seasonal allergies - they can do testing for it just like with humans, but it is very pricey. My vet recommended I keep Benedryl Original on hand, and I can give her 1 to 2 mgs per pound. She weighs 65 lbs, and the tablets are 25 mgs, so I give her two full ones plus a half when she breaks out in hives. (yes,she actually breaks out in hives! It's the funniest darn thing, and scary too!)
Whatever you do, make sure it's the original benedryl, not the one for sinus or anything else, otherwise you could actually kill your dog.