I am not into high frequency much and just know about ringing caused by reflections due to impedance mismatching in transmission lines.

So I recently saw some non-engineer researchers in a lab that they were measuring 3MHz signal with 1 meter 50 OhM coax cable and the cable was directly coupled to the scope input.

In such electronic instrumentation I am wondering when would a 50 Ohm termination required. Is there a practical way to roughly tell whether 50 Ohm termination is necessary? (Assuming source impedance is negligible, 1 meter 50 Ohm coax cable, and a 3MHz signal, scope input impedance is 100Meg)