Using the aapt tool you can check lots of things inside an apk. Use the following command:
aapt list -v -a myfile.apk
this shows hundreds of lines, but you must search the following text "android:debuggable"
this flag can have the values:
0x0: debuggable false
0xffffffff: debugabble true
in my case the entire line is the following
A: android:debuggable(0x0101000f)=(type 0x12)0xffffffff
Oops! My release build had the flag enabled ;)
