Watch whether an app uses TLS
There are Android apps available that can act as VPN service on the Android device itself (even without root). Afterwards all traffic is redirected through that app.

The app captures and displays the proxy, allowing you to detect unencrypted data.

Available Apps:

Packet Capture 

On the step "SSL Decryption" select "skip", otherwise your SSL connections will be intercepted and decrypted.

Apps that use correctly protected SSL/TLS connections will fail while Packet Capture is running and capturing the data.