During nesting season, adult Roseate Terns are recognized by their dark bills and tails that extend beyond their wing tips (when at rest). In flight, they are noticeably lighter than Common Terns, and if their voices aren't drowned out by hundreds of other talkative terns, you might notice their two-noted calls. (Listen to the following embedded sample.)
The Roseate Tern population in Maine is quite low (probably around 200 birds), so unless you're near one of their nesting colonies, you're unlikely to bump into many of them. The northeast population of Roseate Terns is listed as federally endangered, with the main threats to the Maine population being "gull predation, habitat loss, human disturbance, and food shortages" (according to the ME Dept. of IFW). In July and August, small numbers of Roseate Terns from the Stratton Island colony regularly turn up at Hills Beach in Biddeford, which is where I found this orange-legged adult and this black-legged juvenile.