In most of North America, nesting behavior is common any time between April and July. You may not think of it as a field mark, but it can be helpful in the identification process, since it narrows the possibilities.
You can start by noticing the signs of nesting. Almost any bird holding food or nesting material is likely to have a nest nearby. A bird showing extreme agitation, such as a distraction display, is likely to be defending a nest. A bird carrying a fecal sac or an eggshell as it flies from a nest, and courtship feeding, raptors perched side by side, or other evidence of pair bonds can also suggest nesting.