Quoted By:
There are usually some spots where you can squat and be pretty sure that nobody is gonna be coming around but you can't really tell where they are at a glance. If there's a street kid scene in your city I'd recommend asking around there.
Construction sites and houses/businesses that are obviously unoccupied and in disrepair are your best bets if you're just guessing. It's rare for people to come around to these places at night, but if you stay past the early morning you're taking your chances. At some places nobody ever comes and you can stay for years, at others random people drop by unexpectedly.
Outside of squats there are tons of good places to sleep for free, you just have to keep an eye out for them. After a while it even becomes kind of second nature. Rooftops are a personal favorite.