You use the emotional calculator called your brain.
If this does not work, ask someone else.
First you start with this formula: L = t + h. L is Love, t is the number of times a day you tell them you love them and h is the number of times a day you do something nice for them...
No, that's not right...
Love is an inmeasureable quantity, meaning there is no way to measure its amount. There is no need to measure love, it's an infinite supply.