Section 4 of the Maine Citizens Tidal Marsh Guide provides an outline of characteristics that can help you identify coastal wetlands as well as the different values and services that a wetland may provide.
The National Wetland Inventory Maps created by the US Fish and Wildlife Service identify large areas of coastal wetlands, although they were created using 1980s aerial photographs, so existing marsh conditions may be different than those mapped.
Expect boundaries to change. The nature and location of coastal wetlands may change in the future with changing elevation of the highest annual tide due to sea-level rise. (See the Maine Geological Survey mapping efforts for sea-level rise in southern Maine.)
The Maine Geological Survey has simulated the potential impacts of sea-level rise on the coastal wetland boundary for the Drakes Island and Wells Beach, Wells area. Additional online reports and maps for coastal communities in southern Maine will be available in the future. Wetland information is also available from resources on the Maps & Data page.