Technically it just involves a few template arcana, which you can find by googling e.g. has_member or the like. Off the cuff, in the detection code, if I were to write such, I'd just sort of fake-derive from the class in question, and check size of derived class' member.
However, don't do that.
What else to do depends. But it seems like your classes conform to two different "schemas", so to speak, without those schemas being available via the type system (like, it seems the classes don't derive from two base classes A and B). Then one option is to introduce a traits template that tells you wrappers whether the template param T is schema A or B. Specialize the traits for each relevant class that differs from the default. Choose the default so as to minimize work.
Cheers & hth.,
