A stub is just a dummy implementation of a remote object interface delegating the real job using network.
To understand if a class is a businness object or just a stub you have to check its source code and understand its behaviour. If it does businness jobs directly it's a plain class, otherwise, if it just acts like an intermediate to the network layer it's a stub.

For RMI, at runtime you can check if the class is instance of java.rmi.server.RemoteStub, if yes it's a stub, otherwise is a plain class.

Well, a stub is a plain class file. It is just not implementated with the code from the orignal class that it was generated from, instead its implementation is serializing classes, making connections and streaming data. So to answer your question...I don't there is a way you can tell just by looking at the class files. But, I could be mistaken.