DehydRAT (/ˌdiːhaɪˈdræt/) is a simple program that checks if a jar file makes any calls to the Minecraft method to get a session ID.
To use, download the latest version from Releases and drop your jar file on top of it.
At it's current state, the program is trivial to circumvent by RATs and it also generates false positives frequently. Take the result with a grain of salt.