Unless you know where to go and what to look for, you may never see a Glaucous Gull in New England. eBird can help you with where to look (see Glaucous Gull sightings map), and this post should help tune your spotting skills.
A useful starting point is to find a large group of gulls and methodically scan the wing-tip color of each bird. Glaucous Gulls (along with young Iceland Gulls) have white or light-colored wing-tips, in stark contrast to the dark wing-tips of the more common Herring, Ring-billed, and Great Black-backed Gulls.
Size is another factor. Whereas Iceland Gulls are smaller than an average Herring Gull, Glaucous Gulls are larger. For example, below is a photograph of two white-winged gulls who caught my eye in South Portland, ME. The bird, left of center, with wings stretched is a Glaucous; the other white-winged gull is an Iceland.
Another useful field mark is bill color. Both 1st and 2nd winter Glaucous Gulls have bi-colored bills -- pink tipped with black -- like the bird on the right in the photo below. Older Glaucous Gulls (not pictured) have other characteristics, which I won't discuss here as they are less frequently seen in New England than young birds. In contrast, young Iceland Gulls have mostly-dark bills. (Note: The dark-billed gull below is a juvenile Herring.)
At the risk of oversimplifying, if you see a large, all-white gull with a bi-colored bill, you've likely found a young Glaucous Gull. The following photo shows a Ring-billed (top left), Iceland (center), Glaucous (right), and Great Black-backed (bottom).
Of course, at times these field marks are difficult to notice, and size can be hard to judge. In some cases, a particular gull will simply strike you as different. With this in mind, can you spot the sleepy Glaucous Gull in the top photo?