I think it's best to get the AC clean and check in the spring, right when the warm weather starts, but before it's hot. That way the guy can check with fairly typical outdoor temperatures.
Many units have a shut down switch in case of low freon. You should also notice the lack of cooling. Many Trane units I've serviced, it's hard to tell if the outdoor unit needs cleaning. Ask them to be sure to spray on the cleaning chemicals, let them soak, and rinse off with the garden hose. Even if it "looks clean".