Once you have a way to get your two tone audio into the radio, either via injection to the microphone port, or by transmitting an audio file, you can check your linearity. If you have a good double sinusoidal pattern, then you can expect that your transmitter is running in the linear region. Otherwise you can adjust your transmitter, or amplifier and test again.

https://www.youtube.com/watch?v=NapPt2sxBVE