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

Good question. I guess when your pen quits writing.

Rofl, nicely while i develop right into a boy i used lemon juice.. not sure what your utilising yet an simple attempt would desire to artwork. attempt sticking your finger in the ink nicely...if it disappears you nonetheless have some left :)

When the invisible people aren't responding to you anymore

When you see it

The ink would be kinda wet, right...

When the pen doesn't work.

when the pen doesnt work anymore.

when you run out of excuses to not do homework.

less effort in writing

rawr..you are so sexy :) ihihihi. kidding. it is a nice question.