Things to try
→Open ADSB in its own browser (not a tab). I put mine in a different Linux workspace so I can easily switch between planefagging and the board with one key combination.
→Open multiple copies of ADSB in different tabs to watch different locations.
→Zoom to the entire USA and count the number of MIL flights listed; compare with 'normal' for the day of the week and time of day.
→Sort ADSB table by Flag and look for foreign flights, or an unusual number of USMC in the air other than around a Marine base.
→Sort by Silhouette and notice helicopers in unusual places or clustered. Notice unusual kinds of aircraft like Ospreys, Fighter Jets, Refuelling Tankers, Drones (UAVs), Balloons (rare), U2.
→Sort by Registration and look for 02-xxxx
→Sort by Callsign and look for MAGMAxx which is JSOC, usually of interest. Other callsigns also sometimes indicate the nature of the mission.
→Sort by speed and look for planes going unusually fast (>500 kn) or unusually slow.
→Sort by altitude and look for planes flying over 40,000' which would tend to indicate they are in a hurry and avoiding civilian air traffic which generally flies lower.
→If your browser doesn't crash when you try this, on the ADSB Menu, Aircraft tab, select "tails for all aircraft". Then zoom out and look for interesting patterns.
→If you notice someone circling or flying surveillance patterns, zoom in and see what it is.
→Screencap very frequently - you never know when something interesting is going to drop off the map. I use Shutter on Linux for screencaps.
→Learn where the US military bases are and what kinds of aircraft activities are normal in their vicinity.
→Helicopters that have been in the air for a long time are interesting, as are those continuously circling at low altitude.
→Learn the different kinds of planes and their typical missions.
→Pick one area of the world to concentrate on, and become very familiar with what is typical there.
Thoughts from a semi-novice planefag, still learning.