How to enable Inspection, after being Disabled in Android Studio?

If you want to restore all inspections to default, you can go to: File > Settings > Inspections > Click on Advanced Settings

and Reset to Default Settings

Go to File -> Setting -> Editor -> Inspection and mark which inspection that you want to enable.

If your project is under version control, you might notice that .idea/inspectionProfiles/Project_Default.xml and .idea/inspectionProfiles/profiles_settings.xml were either added or changed. You could just reset or remove those files (Making sure to only reset the parts you want to re-enable.)

You can find and disable/enable all inspections here.

For your situation, you have to find and make selected this two: