Possibilities:
1.) The simplest method would be to assume no acceleration or deceleration, and take their last known velocity reading and run with it, so to speak.

2.) Depends on exactly what instruments are and aren't dependent on the ship's power. If they're going at relativistic speeds, a simple spectroscope could give them a good idea of how fast they're going, or they could measure the displacement fo nearby stars. If not...well, things get more complicated.

__________________
"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State