On cartomizers I use a plastic or delrin drip tip as a mouthpiece. I can pull it out and look at the stuffing - if it's gone white on top the carto could take some juice. I always add juice before putting on a fresh battery since they are hotter fresh off the charger, before bed, before leaving the house, when I get home. In addition, any time the carto's vapor tastes weak or the slightest bit wrong, or if the carto is getting abnormally warm, I add juice. I use thin clear juices, around 80PG/20VG, avoiding all dark/cloudy/opaque juices - I want juices that drop no flavoring sediment and don't gunk up the carto stuffing.
You could try carrying a couple of cartos - just switch cartos when one seems like it's getting low and add juice to the other when you get a chance. Or look into carto tanks.
Ahhh - I just saw you are talking about SLIM 510-T? First of all - I would never buy an auto 510. Manual 510's are totally sealed on the end against being damaged or killed by juice leaks. And manuals make it easier to get a good hit. And re carto tanks - that would look ridiculous on a slim 510-T - it would be big and fat compared to the battery. And also - the slim 510-T's own atomizer tanks have crappy capacity - I think 1/2 the capacity of a 1ml cartomizer.