The bottom
coils have gotten a bad rap because people keep trying to use them well beyond their useful life.
What is the useful life? There is no real answer because so much depends on your vape habits (voltage, duration of vape, frequency) and your
juice: thick, dark, sweet juices will gunk up atty's fairly quickly, regardless of the type.
On the Kanger style, you can pop off the chimney easily to clean them better. I clean mine in an ultrasonic bath with baking soda. Some use vinegar or Everclear.
The easy way to tell if your coil is dead is to use an ohmmeter or a
device that reads the resistance of your atty (like a Vamo, ProVari, Zmax, etc.).
You could pop off the chimney while leaving the base in to see if you get your coil to glow. Sometimes, you can revive a poor performing atty by doing pulse burns; however, I would not recommend this on an integrated battery since the pulsing tends to overstress the tiny switches they put in them. I would do a pulse burn only on a mechanical mod.
A new head is $2 and you can get them for as low as $1 if you shop around. I do my own rebuilds. Regardless, you should always keep spares. I have a variety of vaporizers like Kayfuns and Genesis, but my out and about vape is the Kanger style. They are small and swapping out a burnt out coil is a snap and requires no tools.