I have a butt sniffer, which is basically very long thin calipers that can reach over halfway* down a bike frame tube. I was about to make my own when I googled it and found Farr was selling his at a discount, blowout price really, I couldn't have bought the raw materials to make one for less. They're pretty easy to make though, lots of people have DIY versions. The Farr one is pretty deluxe, but other than looking spiffy, it works the same as even a barely-competent DIY would.

*They could be made longer, but they only have to reach halfway 'cuz you can flip and measure from the other end. (Duh.)

They can be used on a seat tube of a finished frame but only from the top, so you won't be able to reach the butt at the bottom unless you make it longer. They're usually used only for checking raw tubes before tacking up the frame.