How did Wanda recognize this character?

Wanda believed it because she wanted to believe it and she was so crippled by her self doubt. Of course Agatha was also controlling him so she was likely manipulating Wanda slightly here as well.

Agatha: I was so patient, waiting for you to reveal your true self. I got close with fake Pietro. Fietro, if you will, but no dice.

Wanda: That was you.

Agatha: No, it wasn’t literally me. Just my eyes and ears. A crystalline possession. Necromancy was a non-starter since your real brother’s body is on another continent. Not to mention, full of holes. But you’re so crippled by your own self-doubt that you believed it.

_{WandaVision, Season 1 Episode 8, “Previously On...”}

Note though that whilst Wanda reacts straight away Fietro is the first to mention that it. Presumably the suspicion, manipulation and his calling it out just set it in stone for her.

Fietro: Long lost bro get to squeeze his stinkin' sister to death or what?

_{WandaVision, Season 1 Episode 5, “On a Very Special Episode”}