My husband says that sometimes you can feel on the outside of the cup where the water level is (by the heat) and recommends that method. Another method we've used is to fill the cup up to the appropriate level with cold water, and then pour that into the device you're using for boiling. That way, when you pour back into the cup, you know you have the right amount.

Very carefully.

But seriously, the two tricks I use are feeling the outside of the receptacle (one should pour relatively slowly), or useing a liquid level indicator. This is a device you place on the rim of the receptacle (glass, cup...) and which gives an intermittent beep when you reach a certain level (close to full) and a constant beep when it's full.

I think you need a $1 (or like $1).

To be honest, I do just like you. I've never been able to tell from the outside of the cup if it was getting full.