See here:
Meet Rayhunter: A New Open Source Tool from EFF to Detect Cellular Spying
The only issue is that it only works with Verizon towers. Never the less, I'm guessing stringrays don't care about the cell carrier so it's possible it'll catch 'em. You DO need a Verizon activated Sim for this mobile hotspot. US Mobile might be the cheapest MVNO (Mobile Virtual Network Operator) that uses Verizon as one of their carriers. Just make sure your plan is Verizon.
Extra credit:
Atlas of Surveillance
Tip #234: Stay off social media, stay of Lexisnexis... I have NO social media accounts. No YouTube, not Farcebook, No X, No Linkedin, etc. I refuse! I have my own website for my resume and whatnot and I control what goes on it.
Edit-
It's not just law enforcement or whoever that has "Stingray" capability. Someone who knows what they are doing can flash a cellphone repeater base station with a certain firmware and intercept phone calls and text message withen a certain radius... It's like flashing a consumer network router with DD-WRT, OpenWRT, etc.
Edit 2-
The article that I linked to mentions how 2G is insecure, but in actuality so is 3G which can be cracked with rainbow tables... LTE is a different animal.