I was outside earlier, and heard an aircraft fly overhead. This is a normal occurrence around here because we live right on top of and next to a few air traffic lanes. How about you? Do you know where your local air traffic lanes are located, and what frequencies they use for communications? This information is important as a change in air traffic, determined by the change in communications level and tempo, may be an indicator, depending on what you're looking for.
Skyvector has online aeronautical sectional and enroute charts that will show you your local air traffic lanes and the communications frequencies used with them. When used with ADS-B ELINT, you get a pretty interesting picture of what's going on above you.