how do you tell when you've run out of invisible ink?

Invisible inks leave a wet trace on the paper until they dry - no wet trace and you have run out

Actually, the kind of invisible ink that I always get.. is the kind where its like purple, before you use it. In the bottle you can see it.. only until when you put it on something [or i guess even in something] then it turns invisible.

That's the beauty of it, you never know. Suprise, suprise, suprise! Geee, you could be waxing eloquent for pages before you realize that you are out of ink and those words would be lost forever. Moral: Don't use invisible ink!

Simple. I tell, 'I've run out of invisible ink'.

Rofl, properly while i became a boy i used lemon juice.. not sure what your utilising yet an hassle-free attempt ought to artwork. attempt sticking your finger interior the ink properly...if it disappears you nonetheless have some left :)

easy your invisible pen drys up

When you can't find your invisible pen

When your x ray glasses can read the coded message you are preparing for one of your superiors.

try to draw some on your fingers. if it feels dry means it has run out. hopefully you dont have sweaty hands.

when your pen feels dry and scratchy on the paper