How to get APK signing signature?

You can access the APK’s signing signature like this using the PackageManager class
http://developer.android.com/reference/android/content/pm/PackageManager.html

Signature[] sigs = context.getPackageManager().getPackageInfo(context.getPackageName(), PackageManager.GET_SIGNATURES).signatures;
for (Signature sig : sigs)
{
    Trace.i("MyApp", "Signature hashcode : " + sig.hashCode());
}

I’ve used this to compare with the hashcode for my debug key, as a way to identify whether the APK is a debug APK or a release APK.