First, find your resting heart rate (best time to do that is when you wake up in the morning before you get out of bed.)
Second, find your pulse and count how many beats you have for 15 seconds.
(Start with first beat as 0) Multiply that total number by 4
(For instance: if you count 15 your resting HR is 15 X 4 = 60)
To determine your target HR:
220 - (your age) - Resting HR x target % + Resting HR = target HR
220 - (age = 40) - 60 = 120 X 60% = 72 = 60 = 132 for target HR of 60% = aerobic training