Your resting heart rate is an important indicator of your overall fitness level. A resting heart rate for "normal" adults ranges from 60 to 100 beats a minute, but athletes often have resting heart rates that are lower - even into the 40s.
I like to use Instant Heart Rate - Heart Rate Monitor by Azumio. Simply hold your finger over your iPhone (or Android) camera and the LED flash will illuminate your finger tip and make it easy for your smartphone to record your heart rate. It saves your data too.
I like to check my resting heart rate 3 times each day: 1) before I go to sleep, 2) right after I wake up, and 3) the middle of the day.