Depends on the area. Obviously the closer to a city you are the harder to tell and the more likely it is private. The backroad Maps gives a good indication the farther away fro a city you are. Most but not all land in the bush (not the country side) is crown.
I camp mostly in the Bissett Creek area which is probably 95% crown with a few hunting camps that got crown leases. as I said the farther north the more crown. Even around the rural areas there is crown I know some that most people don't know of.
I went to a map store years ago and pick up maps of all the rural areas around Ottawa.
In the end it all requires research and sometimes tips from people like me. I'm not going to tell people exactly where I go but will give General areas.
I use to also camp up around Griffith Ontario at a place called Quacken Bush good place you will find it in the Backroad Maps of Eastern Ontario.
One thing I always do is carry lot's of maps, some seem to show different roads. I drive a 4x4 and I'm not afraid to scratch it.

. I carry a hand held GPS, Car GPS, Backroad maps and review Google earth for hours when I set off for a new area to explore.
In fact just heading out tomorrow to explore a area 100 km North of Sturgeon falls to camp and fish over to Capreole and finally to Espanola in 7 day's. Minda ya I'm not Scouting this area for hunting just fishing and camping but you never know maybe Moose someday. Long weekend I will be back up at Bissett Creek.
Last but not least I'm you want to get tips on hunting Ontario Out of Door Forum. You'll see me there.
(Mods I hope I didn't break any rules say the name of the site)
How do you find out what is crown land and what is privately owned?