Stress test at a doctors office while heart activity is being closely monitored is, IMHO, the most accurate way. Either you'll tell the doctor "I've had it" or the doctor will see something on the EKG and tell you to stop. You'll find your maximum HR and figure out your zones from there.
I've had a few stress tests done since my heart attack, know my limit, and set an alarm on my Edge 1030+ when I get to it.