Most batteries the voltage decreases as the battery is used and that voltage can be used to estimate how much life is left in the battery. This is how battery checkers show a percentage remaining.
LiFe batteries do not behave this way. They will show the same voltage throughout the usage of the battery until it is almost ready to drop off to nothingness.
As Four Stroker said, a current monitor sending telemetry data back is the best method. If you don't have that, you can do this: Start with a fully charged battery and do a small number of flights (say for instance 3) and then recharge and look at the total milliamps that were used. Divide that total milliamps by the number of flights and this will give you the milliamps consumed for a flight. Give yourself a margin of error and decide how many fights you can do with a fully charged battery. Good luck keeping track of the number of flights as you use the battery.
Assuming the number of flights on a charge is large enough for a normal flying day, this is much simpler than it sounds. Just charge up at the beginning of the day. LiFe is very tolerant about when you charge and can also recharge pretty rapidly so charging at the field won't delay you much.