This may not get the blemishes out, but I write it anyway.
I would have a serious talk with her about the surrounding topics to her statement, after she has told you her side of the story. Talk with her about how great the new iPads are compared to the old ones, how it's important to take care of one's things, how careful she usually is with electronic devices, how you can't afford a new iPad anyway (if true) etc. Watch her reactions carefully, and check for inconsistencies with her initial statement of having forgetfully left the iPad in the rain. Also, do observe her reactions (voice, body language, etc) to your approach, regardless of what she answers. Finally, IF there is a sufficient amount of guilt, remorse or inconsistencies on her side, confront her with your suspicion and she will tell you the truth.
This might seem like an interrogation technique, which it is. And a very efficient one, too. Remember that even if your gut feeling is not always right, it's never wrong either.