There are various methods. A rule of thumb is 220 minus your age, but that can be inaccurate depending on your fitness level. There are also calculators online

There are various methods. A rule of thumb is 220 minus your age, but that can be inaccurate depending on your fitness level. There are also calculators online that take other factors such as your age or gender into accounts, again being an estimate. Empirically, run up a hill or sprint hard as fast and as long as possible. That will give you a good number. Wear a chest heart rate monitor, these tend to react faster to your changing heart rate and typically give more accurate results than wrist heart rate monitors.