What I meant was how can you tell if all the moisture is gone, that is, the filler is completely dry and no part of it is moist?
I recently cleaned some cartos using almost boiling water, let them soak for 10 minutes, and then used a 10mL syringe and rubber cone (bought from Walmart - the carto hooks to the end of the rubber cone, which is hooked to the syringe) to completely flush the excess water out (plunged several times back and forth - draw the water in and plunge it back out) from both the threaded end and the mouth end. Then I plunged several times without drawing in any water, which helps to get rid of any excess water, and then I blew through the mouth ends and let them sit by a fan for 24 to 30 hours. Then I refilled them with liquid, and from then on, whenever I vaped those cartos, I would have a problem with having to spit out water that was on my tongue after each drag. It took me a while to realize that this problem was due to the cartos not being completely dry before I refilled them.
Anyway, now I leave them sitting by a fan for several days after cleaning them and before refilling them (I'll probably let them sit for 7 days or so). And I no longer use boiling water or near-boiling water, because two of the 11 cartos died after that -- and the filler shrinks every time. Now I just use hot tap water when flushing them with the syringe and rubber cone (which is the best method I've found to clean cartos so far). I got the idea from watching a YouTube video a few weeks ago, then srcowell started using that method, and he gave me a couple more ideas on how to do it the right way (that you can draw water into the mouth end of the carto from a bowl or pan, then plunge it back out into the sink, and repeat this several times -- and then you do the dry plunging several times to remove any excess water). Blowing through the mouth end as the last step will sometimes remove the last little bit of water.