A combination of pattern recognition and common freaking sense. If you think antifa could possibly keep their noses out of the rioting, you're seriously deluding yourself. It's their modus operandi. But besides that, plenty of people, and groups of people, who give off the air of antifa (eg. weak looking black clad white males) can be seen in various videos, along with stylised-A graffiti. It's that same pattern recognition and common sense that says that the umbrella guy is probably a false flag plant, since he gives off a totally different gestalt than an antifa member despite some surface similarity.