RPIC / Visual Observer
The attentiveness of the operator and VO is an important first factor in monitoring the surrounding airspace.
Also having a plan for how the pilot will respond such as pausing the mission, going straight down, and putting the camera straight down. A laser range finder if available can be used to gauge distance the ground as well if terrain has significantly changed to where the drone is.
Potentially have stricter standards regarding RC strength status to avoid a RTH or lost link scenario as well.
Airspace Monitoring App:
Monitoring using tools like ForeFlight is useful for better awareness of planes that may come into the operational area.
Built In Tools with Drone/RC
DJI AirSense utilize the ADS-B IN built into the drone to receive ADS-B Out signals from manned aircraft that are broadcasting and notify the operator..
Pilot 2 will display live information on the camera view, compass, and map, along with a verbal warning and on screen alert. It is always "on" and important to monitor alerts.
Remote ID / Drones
For drones a hardware/software system such as UAS Sentry could be used that monitors Remote ID: . Have also seen DroneTag announce a system.
Multi Team Efforts
Coordinate flight plans to avoid flying in the same area.
Keep radio communications.
If both operators are connected to Flight Hub 2 can also see where the other team is is operating on the map of RC or browser.