Something they might have is a lot of strain gauges around the rocket. They can tell if the rocket is bending and compressing (and how) -- but they are also essentially high-speed microphones. Valves make sounds (cause vibrations) and they are easy to spot if you have high-speed samples of your strain gauges. You can also triangulate distances if you have multiple strain gauges with synchronized high-speed samples.

It's good practice to dump data into several flash memories + transmit live. The live transmit is likely only a small summary of all the data that they capture -- distributed flash memory locations is a cheap way to increase the chances of getting useful data after a boom/crash (put them in orange water tight containers that can float + give them a beep-beep radio transmitter).

Telemetry?