In the database you can check for the package "cmxue" which will have the user exits.
I believe you use option of User Object Registry as well which will be in MDM hub console.
Instead of checking it from backend, you can check it from console.
If you really want it to check from backend, then you can follow above answer.
HUB UserExits can be found in CMXUE package
For IDD userexits export the application and get the JAR.