If you burn the cotton wick by firing the mod without any juice on the head, then it's toast - it'll never vape properly again, because of the burnt cotton taste, unless you can rebuild it.
If you manage to never vape your cotton wick bone dry (and you shouldn't ever really, you can taste that the wick is getting dry a good few firings before it's *bone* dry and you damage it) you can tell when the head needs changing either by the muted taste (but that *could* be vaper's tongue) or by looking at the wick and it's all crusty, gunky and black-coloured.
That's why I've never used tanks with factory heads now for years - they cost a lot, you always run out of them at inopportune moments (if you're me), and the Quality Control on a lot of them is atrocious.
I used to buy cheap, cloned Protank heads so I could rebuild them myself with nice coils and Japanese cotton and done right - I saved a lot of money that way.
That's what I always disliked about the Nautilus - lovely tank, especially the Mini, but the coils don't last very long (especially with certain juices), they're expensive, the clones of them are generally rubbish, and they're such a bugger to rebuild that I gave up.
These days, I just stick to my trusty Taifun GT2v2, my Bellus and the Flash-e-Vape v3 and Kayfun v3+ that I've got arriving soon...
