For the past month, our dryer has been tripping the breaker switch in the circuit panel. Initially, it would do it once a day, then a couple times a day, then several times during a single drying cycle, now every 2-3-4 minutes during a drying cycle. Then, every once in a while, it will go through an entire cycle without tripping the breaker. I've checked the exhaust vent all the way through the dryer to outside the house and I've checked the wiring in the back of the machine (no loose wires). Is the problem a bad breaker (what amp should the breaker be?) and, if so, how do I tell, is it a bad sensor in the machine (and how do I tell), or what other things can I look for before calling a tech guy out?

The only way to test a circuit breaker in your case is to replace it. The breaker amp rate will be on the back of the machine.

Note the electric element could be intermittently shorting to ground blowing the breaker also.