If you have corrosion and resistance in the connection, the circuit breaker will not trip. You can have a fire while drawing 20 amps through a corroded 30 amp plug connection. The breaker trips on current overload and not heat in the inlet.
This is true. A 30 amp breaker will not trip at 31 amps, it takes a significant fault current above 30 amps to trip it.
I put my hand on the plug body to check the temperature and wiggle it to make sure the clamping ring is still tight. Sorta like checking trailer hub bearings for heat when you stop to fill up for gas on the freeway, quick look at the tires, etc.
Regarding amps: I got one of these and it has turned out to be one of the most used tools on the boat, it measures AC and DC amps up to 100 amps, and is so sensitive to 2 or 3 decimal places, making it useful for tracing down small currents too. Compact and fits into the small spaces on the electrical board...
Uni-T UT210E True RMS AC/DC Current Mini Clamp Meters w/ Capacitance Tester
Cut and paste that into Amazon or your favorite site.