Secondly, be careful with the wood if you don't know precisely what it is. For instance, making charcoal (or even burning) pressure treated wood is a horrible idea.

Conversely, some randos see an open dumpster and think its a great place to chuck their old car batteries and whatnot.

You just never know though. For instance, it could be thought that there isn't any asbestos in the home being renovated. But maybe there were a couple pieces of drywall or pipe insulation that happened to have asbestos and it wasn't noticed. It happens.

There is also a very good chance that there's lead dust and chips from old paint or pipes that's being thrown in as well. Probably don't want to make charcoal or anything out of that or any new SPF/Doug tossed in there that maybe got some of it on it you don't notice.

As for pressure treated wood, it is pretty obvious what has been treated and what has not, at least in my experience. Not sure if it's the process, or if they intentionally dye it, but treated wood has a strong green tint, though could probably have other colors too depending on the treatment method. But fresh untreated pine has a pretty obvious color.

Be careful if it’s been sitting in the sun. Lived in a building that had its hot water recirc lines fail one after the next like 5y after construction and I suspect that was why.

So it's always better to ask first, otherwise it's technically stealing.

This is real world-illiterate textbook advice.

You can't ask because then you're pushing the responsibility of you being there onto someone else. They don't want some ambulance chasing lawyer to come bitching at them if you get hurt so they have to say no. But if you're discreet can just pretend that you're trespassing.

So just don't ask and know what is or isn't fair game.

Additionally, scrap of the mixed random stuff variety that is worth picking through is cents a pound. They won't care if you're not taking serious volume.