Many of those little access roads you see in Ontario are on crown land which legal to camp on. Same thing out on the West coast.
The trick of course is to find one far enough off the road thats both suitable and invisible to ppl from the hwy.
I've been known to go 'Commando' camping a few times. I've never had a problem. Thats not to say it isn't. I've also found a nice spot that is most definitely private property but asked the owners and am rarely turned down. Most of issues are with you lighting a fire. So if you can, mention that you aren't planning on it (I travel with a small cookstove and use it mostly for boiling water - morning coffee and instant oatmeal).
If you go the private property route, make SURE you clean up after yourself! If you can, leave it better than when you arrived.