Kingcycle or a lab rat test will determine your HR accurately in controlled conditions. However, without access to that, to get a good guide, try and find a hill that's a good hard shortish climb that gets you out the saddle - but not long enough to ride up in a rythym. (say half to three quarters mile?) Have the HRM on, make sure you're fully warmed up and....

Ride up said hill as hard as you can in the biggest gear you can and coast down, turn round 3 times and repeat. keep an eye on your heart rate max.
Fourth time up pay extra attention to the HRM, you will no doubt max out on that effort or the one before - Not as good as a kingcycle test, but it will give you a much more accurate reading than the 220- rule.