Red-tailed Hawks are the most common soaring hawk in New England, but in some areas Red-shouldered Hawks are also quite common. A reliable way for me to tell them apart is by ear. Red-tails give a loud keeeeeeerrrr call, which lasts 2-3 seconds, while Red-shouldered Hawks give a shorter kee-aah call, which is typically repeated several times in quick succession.
Lacking audio signatures, these birds can be separated by appearance. If you notice a large soaring bird who lacks a red-tail, check near the wing tips for translucent arcs (which appear as light patches in the photos, but are often more see-through in person). While not always obvious (lighting can be a factor), these field marks (called windows) are a good clue that you've spotted a Red-shouldered Hawk.