No fences. No signs. Hell, no doors.
Check it out! If anybody says anything, play dumb. Tell them you (based on your age) are/have gone to school for architectural engineering and you find it a fascinating structure and ask if they have any background info. Go in with a camera around your neck/in your hand and no bag or anything looking like you might be stealing/vandalizing the place. Anybody inclined to look out for the place will be happy to talk to you about it.
I'm the biggest wimp for hopping fences or anything like that, but this is just so inviting! Nothing to worry about here