Thanks for writing. Grub-infested lawns are most apparent in late summer when sod starts tearing in shreds. You'll also find white C-shape grubs in great quantity when you dig.
There are a number of effective grub-killing products on the market, under brand-names such as Bayer, Grub Ex, and Grub B Gon.
If you prefer a more natural approach, there are organic products, as well. For example, a company called Gardens Alive! sells a type of microscopic worm that attacks and kills grubs.