There is a proprietary library whose API I want to re-implement, which is provided to the users with specification, and its use is limited by its license to internal evaluation (to develop programs that will be ran by that library) and independent implementation of the given spec (which means it should not derive from/contain any source code or binaries of the provided library).
Let's say, before deciding to make my own independent implementation, I searched the web and found some programs that seem to have already made such implementation, and are open source. My initial wish is to go ahead and fork them, but I want to make sure those implementations are indeed independent and aren't stolen from the copyright holder (directly or by decompiling the binary, which is easy for Java, for example). Is it possible?