You should try to prevent smouldering when using fire wicks at all times. Whether it be staff, poi, fans or any fire Kevlar related wick products.
Once the wicks start to flicker near the end of a burn, then that is a sign the fuel is about to run out. At the flickering stage put the flames out just before they go out by themselves. This will help to prevent the wick from burning and smouldering.
If you let the wicks go out by themselves the wicks will then burn and smoulder, potentially damaging the wicks.
An indication of smoulder damaged is the wicks can fray easily and quite often turn white on the smouldered edges. If you see this then that is a sign of the Kevlar wick material burning after the fuel has been used up.
This link below contains some helpful tips in regards to caring for wicks:
How to make your wick last longer