How do you determine the Android Architecture during runtime?

By "library" you mean a C# library? or a Java library? how are you loading it? you should be able to call that Java code from a Unity script, something like this (didn't test so bear with me): using UnityEngine;

 public static class AndroidHelper
 {
     // Returns the system architecture
     public static string GetArchitecture()
     {
         using (var system = new AndroidJavaClass("java.lang.System"))
         {
             return system.CallStatic<string>("getProperty", "os.arch");
         }
     }
 }

Let me know if this helps :)