Fitness level doesn't have much to do with the max either. It's simply how fast will your heart beat.

If you're interested in finding out, buy an inexpensive HR monitor. Then find a long gradual hill. Warm up and start riding up the hill at a good cadence. Every minute shift to a more difficult gear while trying to maintain the same cadence. Once you can't keep the cadence up, shift again and perhaps once more until it's difficult to remain seated. Then shift one more time, stand up, and sprint all out for 20 seconds. That will be your max or very close to it.