Only real way to know it to get tested. I don't think I'd rely on the online calculators because you have absolutely no clue what formula they're using or who came up with it. The best way is to go get a VO2 Max or Submax test.

That being said, the most commonly used formula is 220-Age - but accuracy here is said to be +/- 10-15 beats. You may look into the Karvonen method which is a a method of determining heart rate ranges for varying levels of effort. It still uses 220-Age as a base however - but it improves by using resting heart rate as well.

Very good point!

I would say if you're really concerned about heart rate or you're doing heart-rate based training, then you really need to go get tested. Otherwise there's really no reason to even worry about your heart rate.

I use a heart rate monitor when I train.  I'm 42 so my predicted max heart rate is 178 bpm.  There is definitely a margin of error with the 220-age calculation.  In fact, my heart rate has gone as high as 194 bpm during high intensity (but still submaximal) interval training workouts.

I still believe the Karvonen equation is appropriate for beginner and intermediate level exercisers but I choose to use the measured value of 194 bpm as my max HR instead of the predicted value of 178 bpm.  As a result, my training heart rate range (70% to 85%) is 153 to 173 bpm instead of the more conservative 141 to 160 bpm.

I also use Rate of Perceived Exertion (RPE) to assess workout intensity and recovery (0 = nothing at all, 10 = very, very heavy).  When I run at a 10 min / mile pace, my heart rate is 164 bpm with a perceived exertion of about a 6.  At an 8 min / mile pace, my heart rate is 178 bpm with a RPE of 7 to 8.  That seems about right.