I have different colored end pieces (from Smokeless Image) that help me. Red for my Sweet Swish (cherry), dark blue for grape, light blue for blueberry, etc.
When I refill, I put the condom back on. I fill about 10 cartos at a time which last me 2-3 days. If the condom is on, I know it's filled. when it's empty, I leave the condom off.
And I have never used the word "condom" that many times in one sentence in my life.