Buffy wasn't so easily fooled, and her expression clearly conveyed that as she stood there staring at the other Slayer. She wasn't being judgmental or scrutinizing, just concerned, and even if she knew Faith might not like it, it was just Buffy's way.
"It's good that you're here, then," she said, not really voicing what she'd been thinking so directly. "It'll be easier to look out for you and get them off your ass." Literally and figuratively.