I assume you're talking about handlebar clamp diameter - not the circumference. You can measure, but as DeadSailor said the measurement is often stamped on the bar and on the stem (sounds like you're starting out with the bar).
But here's the most common sizes:
25.4mm - nearly all flat bars, older or cheaper drop bars
26.0mm - most better-quality road bars
31.8mm - newer "oversize" for both road and mountain bars
Others: older Cinelli drop bars are often 26.4mm. Moustache bars may come in either 26.0 or 25.4, as do bullhorn bars (though most bullhorns are 26.0).