After a while you get a feel for the country that comes through from the map.
E.g. In Willmore Wilderness, you will often find hunting camps that are clear and pretty well made, but are only used for a month a year. They will sit at the base of a pass, or the junction of two creeks (better grazing for horses)
Since I am blessed to be able to use a fire for cooking (low traffic) I look for places where the timberline comes into the meadow and is close to water. Failing that, where a ravine large enough to likely have water comes down to where my trail will cross it.
In a different context, when traveling by canoe in Northern Saskatchewan you learn that the top end of many rapids is a good place to camp. Some obstacle backed the water up, and so it's likely to at least be out of the bog. The bottom end of a portage is often boggy.
You also will look for points of land that catch the wind if the bugs are bad, and otherwise just look for places where the tree cover is less solid. (Upwind end of an island is great.)
My usual habit is to start looking in mid afternoon. If I'm seeing reasonable looking spots fairly frequently -- several per hour, then I will push on. However if good sites are scarce (Mirror River, SK, I'm talking to you.) then I may stop early.
The map will tell you what sort of country you are in. When you are in a new type of country start pretending you're looking for a spot right away. You'll pick up what features on the map give valid hints.
The gotchas are when the terrain changes. E.g. We had been doing puddle jumping and portaging from Cree Lake toward High Rock Lake, on a chain of winter roads. Would have been easy camping, (but not pretty) at any point where the winter road met a lake. (This is not always true. Winter roads are often frozen bogs, and terrible in summer) But we had to turn off that route and pick up Browne Creek. That was a mosquito farm. Browne Creek was 2 feet wide, had a corner every 6 feet. We portaged over a mattress of wet moss trying to keep from hitting the canoes on the bog spruce and tamarack. 2 km took us 4 hours. Yuck. Fortunately we only had lunch in there. Peanut butter and bugs.)