The only way to really know if it is out of e-liquid is when you start getting dry hits. When I say dry hits, I don't necessarily mean those horrible, burnt, make you want to cough up a lung, type of dry hit. Rather, the vape is just not as saturated as it should be and tastes a little off. This does not ruin a cotton wick because there is still e-liquid in the wick. As long as you are not taking lots of dry hits, anyway. I am using that exact model, and a Tobeco model, with a nano kit. However, I rarely look at the tank to see how much e-liquid is left.
I like a loose draw so, I removed the airflow control screw from both of the kfl+ clones. Because of this loose airflow, my normal draws do not properly pull e-liquid into the chamber and I get a few dry hits on each tank full. When I get a dry hit, I do not normally look at the tank to see if it is empty. Instead, I take a few quick, sharp, unpowered, draws and wait a few seconds. If there is any e-liquid left, it will saturated the wick and give me a properly saturated vape. If I still get a dry hit, I reach for my bottle of e-liquid because, I know that the tank is empty.
I hope this long explanation helps. I can be long winded at times. ;-)