X 2 on the hay fields, and X 2 again on the hills.
When the fields are mature, it takes a lot of glassing to find the mounds, and even then the little brown buggers can disapear into the grass in a second. The best time to shoot them is just after the hay is cut. Depending on the amount of rain in a summer, you can get 3 cuts, but you're guaranteed 2. First cut should be coming off soon, if it hasn't already. I don't know, I'm not in North America right now. First hay is off by July, second by august, and third around the end of august.
After the fields have been cut, it's a wide open space of 6" high stubble, and you can see every mound clear as day, and watch the little dirt squirrels running around. That's the perfect time to go in & grease them.
There are several things to be aware of when shooting ground hogs. ALWAYS, be aware of your back-drop. This will be the farmers biggest concern, especially if you're using anything more powerfull than a rimfire.
Another thing to look out for is rules & regulations on calibres that you can legally hunt small game/varmints with. A lot of the more densly populated regions of Southern Ontario have regulations on legal calibres.
Happy hunting!
Tim