detecting if a class has been changed

I have two java API project X and Y, the project Y depends on X by using some classes declared on X, and these two projects are running on two different servers.

Now let assume that a class C of X has been changed (e.g we've added a property, changed a method ...), how can Y detect the change? is there any way or workaround to do in order to detect the changes? maybe using java reflection or using some rest calls From the API Y to X to check periodically the status ?

Thank you