You would seem like the expert (and x-pert) on the following topic: how do you tell the difference between a picture of Matt Murdock and Scott Summers in the wild?
Assuming that you’re talking about the comics versions, hair color is the gold standard. Matt’s is red; Scott’s is brown. Unfortunately, that’s dependent on lighting, and they’re often both colored with reddish-brown hair, so here are a few other varyingly consistent clues:
Glasses Shape: round frames usually mean Matt (I can think of one story where Scott wears them, but he’s fully decked out in ‘60s clothes and running around with Emma Frost, so it’s pretty easy to pick out); and wraparounds or sports-style glasses pretty much always mean Scott.
Accessories: Scott often has an X-Men or Xavier School logo somewhere on his person, even when he’s not in costume. Matt carries a white cane and often a briefcase.
…of course, then sometimes you get a panel like this; and, well: