Not sure about the ones the TSA is looking at, but a company in the UK that makes them has a video on their site showing a unit with this description

The Kromek Bottle Scanner is a desktop inspection unit that can quickly verify or classify liquids, gels or aerosols based on multi-spectral analysis of x-rays using a combination of Kromek detectors and the proprietary ADMIT algorithm.

The system scans, analyses and classifies liquids in under 30 seconds, with a high degree of reliability and accuracy. The system can handle various shapes and sizes of sealed glass, plastic and metal containers. Additional materials can be added to the system database and the user can specify these as required.

They put the bottle in, press a button on the screen, and it says pass or fail once it's scanned.

Datasheets and more here - http://www.kromek.com/

It seems like a pretty cool machine for not a lot of money (though you can be sure the TSA will spend way more per unit)