You can normally tell if the flavor decreases or it starts to taste gross or burnt.
And you can also tell when the amount of vapor decreases.
With experience, you will kind of know in advance when you will want to replace the coil. With the Kanger T-2s I replaced them every 8 days. Protank heads about 12 days.
With the RBAs I am using, I can tell it is time to dry burn the coils and replace the wick when the resistance drops by 0.2 0 ohms.