It's much tougher now I agree. In the late 60s when I realised my interest in WW2 airfields, I would wear my RAF uniform at weekends during my outings and carry a clip-board. Contacting local landowners in person and more often than not got approval to walk their properties. If there was resistance I might imply that a visit to follow-up illegally buried caches of ammunition was possible (notice all the ifs and maybes!) and that I was the reconnaissance. My code was, always be polite, dress respectably, always get permission, and check-out before leaving.

Ideally (nowadays) you can be armed with plans and drawings and some knowledge of the function of remaining structures and brief the owner on your intentions.