The failure of the Falcon 9 first stage landing during the [Eutelsat/ABS mission][1] was attributed to: 

> [thrust was low on 1 of 3 landing engines][2]

  [1]: https://www.youtube.com/watch?v=gLNmtUEvI5A
  [2]: https://twitter.com/elonmusk/status/743097337782763521

which indicate that engine thrusts are part of the telemetry data.  
I can see how the thrust can be measured on a test stand.  But how can they be monitored during the flight?