The first idea that pops into your head is that you could just confront Ennis and try to get her to spill the truth, either by being upfront with her or finding the right button to push. Otherwise, you could watch her from afar before she sets out to Burbridge and try to get a sense of what she's thinking based on her actions. If she caught you, you might be in a whole new world of trouble though.
Maybe it's best to avoid Ennis altogether and see what Caleb has to say about it. Caleb's a tricky one. Like Rex, he cares a lot about popularity and being seen as "cool," but you might be able to win him over or let him know you're not to be messed with. Ashley might be another option.
"Hey Diego, do you know if Ashley is still spending time with Ennis and her family?" you ask. "I know she used to hang out there a lot."
"I'm not sure, and actually could we leave her out of this? It's been hard enough for her to move on with her life. I'm pretty sure I can find out more by digging around on the Internet than we could from asking her questions anyway."
So Ashley's off the table, at least for now. That still leaves you with several options.