Hack Ruins are custom buildings made entirely or mostly made out of barricading elements, most commonly stone walls due to their size. As the forts are large, it's usually exploiters usually responsible for their construction. They almost always come with powerful and rare weapons such as the Patriot Assault Rifle, MK-48, Mk 17, and M249. They are quite rare and for most, just a fantasy. These can be common in player hotspots or in corners of the map. Nowadays these are almost impossible to find due to the fact they patched exploiting.
A good way to tell if they are abandoned is to look at the barricades. If the hacker is still there, all of the barricades will look deliberately placed, and placed well. If the hacker left the server two hours ago, and people with entrenchers have come by, you will notice that not only are there holes in the defenses, but random nearby areas will have barricades and possibly items strewn about them in a more or less random fashion. Some of the weapons found, if any, will also have very little ammo if it has been raided. This is usually because someone with, say, an M4A1 came by and saw a hacked in SCAR-L with ammo, and took the ammo without the gun.
If you find an abandoned hacker's den, loot until you're satisfied. Unless it's in a hotspot, somebody probably won't be tracking you. If you're satisfied for the day, find a safe place to log off, tell everybody where the ruins are, and log off (just a nice thing to do).