How do you know when your pen with invisible ink is empty?

When it's empty, the cartridge becomes visible. doh. otherwise how else would you know?  

When the writing is on the wall

My penis IS empty when there no longer is any ink.

Hope that helps.

It becomes less in weight.

When the "nib" is dry

(you can tell by the feel of the pen as it moves over the paper)

That's how they get you. They'll tell you that you need to buy more ink even if you don't. Thats why I don't purchase invisible products. It's just a headache waiting to happen 

I have pen with invisible ink;I have pineapple.