A reliable/fast way to know an APK file's api level
I googled quite a while and found the following way is the best:
If you have Android Studio installed, using the "Build > Analyze APK" menu command is indeed a convenient way to quickly determine the supported API level of an APK file. Here are the steps:
Open Android Studio.
Go to "Build" in the top menu.
Choose "Analyse APK..."
Navigate to and select the APK file you want to analyze.
Android Studio will display information about the APK, including the minimum and target API levels.
Surely if you don't have Android Studio installed, then this way cannot help.