Choose a period of time (say, three months). Take a section of hair. Measure the section from root to end with a ruler at the beginning of the period. Measure your hair from root to tip with a ruler at the end of the period. Take the difference between the two (average is 1.5" difference over three months) and divide it by the number of months in the period to get your average growth rate.
Ex: If you have 12" of hair today (measuring with the root to end method
not the lay-tape-rule-along-your-scalp method) December 1 and then measure April 1 and find that you have 14" of hair, then your growth rate is (14 - 12)/4 = 2/4 = 1/2" per month.
Make sure you pick a long enough period of time. I know people talk about getting 1" of growth in 3 days but realistically, you want to pick a period of time sufficient for any length differences to be clearly due to growth, and not how well you flat-ironed your hair, how much new growth you have (assuming you relax), or how tightly you pulled the section of hair.
Make sure you measure in the same area too. I don't know about your hair but the hair at my crown is 3-4" longer than the hair at the back of my head. So, if I measure at the back and then measure at the crown, it might seem that my hair has grown 3-4" in just days if I don't use some gumption.