My call as a noob is that if pants have a distinctive pattern, a pin or chalk stripe, a check or plaid, they very well may be part of a suit.
This is stuff that happens in a thrift. A jacket that has pin / chalk stripe, plaid, distinctive tweed - get over to the pants rack and see if we can find the other half. Pants, the same thing. The orphaned half never looks right wit de jeans / blazer.
Use your eye. If the pants are grey flannel, they may be part of a suit. Go look for the jacket, but if it's not there, who cares? It's grey flannel pants. Main thing, if they fit.