Confront.

You sit down on the bed, picking up Jenna's phone to hand to her. "You got a text while you were in the shower."
Jenna walks over, hair wrapped in a towel, and looks at her phone quizzically. You watch her face as she checks the text.
There. A flicker of something crosses her face. Eyebrows up and together, eyes widening slightly. Fear. Quickly she relaxes her face, but there was no doubt that it had happened.
"Something important?" you don't overplay your hand. Give her a chance to either explain, or give her enough rope to hang herself with.
"Just a friend of Hannah's. Met him in Vegas. We danced a couple times, and stupid Hannah gave him my number. I've been meaning to block him. "
You shudder a little inside. Just enough truth to be believable. You probably would do the same if the roles were reversed. She's attempting to be nonchalant, but her movements while dressing are a little too mechanical, a little too slow. She's trying too hard.
"Oh yeah? I was wondering if any guys had come on to you in Vegas. It is a party town. It's only natural, right?" you fight very hard to keep your tone under control.
She laughs, but it sounds a little hollow. She pulls on black lace panties and clasps her matching bra before donning an orange and cream dress with floral patterned skirts.
"What happens in Vegas, stays in Vegas, at least that's what they say." She turns to regard you. Is....is she studying your face to see if you believe her?