How can I tell when I've run out of invisible ink ???

stick the pen in your eye if it hurts and burns you have ink....if it just hurts go buy some more ink.....oh and stop by the doctors office on the way to the store

Ask George W. Bush - he used invisible ink exclusively to sign all his vetos.

You will know when the invisible writing runs out

They run out every 6 years exactly, so make sure you have a spare cartridge!

I find they work better in invisible pens too.

Rofl, well when i was a boy i used lemon juice.. not sure what your using but a simple test ought to work.

try sticking your finger in the ink well...if it disappears you still have some left :)

Good Question! MMMMMMMMmmmmmmmm ,Huuuuuuuuhhhhhhhh, Hold up the paper to the light.

Let r wr t g st ts t b c me di f c lt.

You can't but the reader will know

everyone starts replying rather than ignoring you for all those years!!!!!! (well i presume so anyway?)!!!lol

when you can see what you have wrote.