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

The eternal question!

You have run out of invisible ink when the ink becomes visible...

You can see what you wrote, or if you're using a printer, you get an invisible message that your ink is low.

When you stick your finger in the bottle and cant feel the ink anymore.

Rofl, nicely whilst i became right into a boy i used lemon juice.. undecided what your utilising yet an hassle-free try would desire to artwork. attempt sticking your finger interior the ink nicely...if it disappears you nevertheless have some left :)

The invisible indicator light will flash and you wont see it

when your writing indentation becomes indelible.