While using an aircraft radio to monitor aircraft activity, the FAA strongly discourages unnecessary transmissions on aeronautical frequencies.
You can listen, but you cannot transmit unless in an emergency
A handheld radio must be licensed when not used from within an aircraft or under the license of an airport UNICOM radio. The one non-emergency use of a handheld is talking to a flight student who is on a solo training flight.
As with any rule, in the event of an emergency, you can break a rule but you better have a good reason when the FAA calls.