No, you can’t tell - you’re making assumptions. That’s exactly the same logic people use to justify shaming trans people, Muslims, or anyone else who “looks” like a stereotype they fear. Everyone convinces themselves their stereotype is the special exception. If you’re okay with that logic here, then you’ve basically conceded that appearance alone is fair grounds for public shaming across the board.
And the ratio of people dressing like this to actual mass shooters is so vanishingly small that your assumption is going to be wrong virtually every single time.