Go to y'all's carrier and ask for a detailed report of texts incoming[don't share. We wouldn't want a break up, would we?] with the phone numbers/emails* listed. If they say they can't, look them in the eye and tell them why, then ask again. They'll refuse, claiming too much paper and yadda yadda yadda. When they finally budge, look at the timestamp on the messages received, and compare it to the bill statement details. You'll be able to find if it's an ex or a shot-down hopeful. Or y'all's parents trying to be tech savvy and trying to get y'all to break up. Hope this helps.
Seriously... Don't share the incoming text details. Don't offer. All good relationships are built on lies. Or so I heard in a movie... I think...
*It's easy to send an email to a phone. For me, it's my-phone-number-here@messaging.sprintpcs.com If someone knows my phone number, and wants to send me a text from, say, the white house receptionist's main line, the can compose a short email[160 characters], spoof the email address so it appears to be a phone number[the white house in this instance, could be your little brother], then send it to yourphonenumber@email.format.yourcarrier.com. There ya go.
Again, hope this helps.