How do you measure volts at a circuit breaker?

If you have to ask, call an electrician.

Seriously, if you poke around inside a live electrical panel where the breakers are, you have the potential not only for a fatal electric shock, but also for an arc blast. To do this safely requires proper protective equipment (blast shield, gloves, boots) as well as properly rated industrial voltmeter with the right kind of input protection circuitry. If you've got the right equipment and the safety training to know how to avoid killing yourself when you use it, then you'll know how to measure the voltage (probe the output of the breaker against ground and read the number on your meter).