Hi vapors

just got my mega dual coil cartomisers for my ego-c cause i was having nothing but problems with it (all sorted now btw). I really like them. i got some cola flavour with it and using this through it gives me a huge throat hit kind of like getting a live grenade stuck in your throat
The question i have is, i know its impossible to see how much liquid is left in it but is there any kind of indication you get that tells you its time to top up again like loss of taste or lack of vapor? last thing i want to do is burn it out.