Hi zrux and wellcome to LQ,
the find-command should work for you. Look at man find and especially for the -perm option.
find . -perm 770 -print
will find all files with this permissions. You'll have to find out then which files are not printed by the above command.
Another way to make sure that all permissions within a directory are equal to 770 would be to use chmod with the -R option (look at man chmod).