Angel would've known.
It was wrong, and it wasn't fair to Riley, but that's what Buffy thought whenever she looked at her boyfriend.
Angel would have seen right through Faith, been able to tell that it was her just by looking into her eyes. She may have worn Buffy's body, but there was more to a person than just what they look like. Angel would have known that as intimately as Buffy did.
Angel wouldn't have slept with Faith. He would've known, explained to the guys, and come to Buffy's rescue.
Buffy decided that she needed to see him, badly.