It's all too quick, you think. One second, you're in the Bennet house with Elle... and the next you're on some beach. You don't even see Elle before you're transported here. You're not sure where she is, really. And then you look ahead and see her there. She's still wearing your shirt, she's still bleeding a little from her leg, but something's wrong.
You notice instantly that she doesn't have that same buzzing static noise she usually has; her electric power that you both now share. It connects you two, the same way you were connected on the floor in Stephen Canfield's house. She had the quiet popping noise in the Bennet house as well- her power almost daring Mama Bennet to try something stupid.
But here, you can't hear anything. You think for a moment that it's the ocean drowning her signature static out. The two of you kiss, and that's when you're positive something is wrong. This is not Elle.
You decide you'll play her game. You lay her down in the sand and take her tiny hand in yours. When she doesn't squeeze your hand the was she has been, it cements your suspicions. This person is an imposter.
You decide to find out who they really are. A few seconds later, you know. You've come to decide that Mother has a little explaining to do... after you test out your new shape-shifting power, of course.