Look for the AMC Adjustment point (modulation potentiometer) or modulation adjustment pot. Use a reference for your brand and model of radio to identify the correct potentiometer.
You will typically turn the adjustment counter-clockwise to increase the modulation. You can start by turning the adjustment counter-clockwise until it stops to achieve full modulation.
At this point you will want to speak into the microphone and if the reading constantly peeks you are probably over modulated. Back off the modulation until speech is no longer distorted.