That would work too, though there might be a few ambiguous possibilities to resolve unless you knew details about quite a few binaries.

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Forming opinions as we speak

Use the interstellar medium to gauge your speed, and triangulation with the local stars to find your location. That is the equivalent of both a log and a sextant; that should give you a fairly good idea of how long it would take to get where you are going, if anywhere.

That does assume that you have some instruments, but they could be as simple as a telecope mounted so that the angle can be read, and a thermometer to take the temperature of various parts of the hull. Friction with the interstellar medium will heat one half of the spacecraft; at high speeds this heating will be considerable, despite the low density of the medium.
Oh, and you need astronomical data, too.

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New Orion's Arm Site . The Starlark . Against a Diamond Sky (OA Novella Collection) . OA Flickr set

Actually I wasn't. The question in question entered my head while reading OTHER science fiction.

Thank you gentlemen for all the information and ideas!

Unless you are headed toward some particular place, such as a star that
you don't want to run into or a rendezvous point that you want to get to,
or you are trying to get away from someone who is chasing you, I don't
see much reason to know your speed (relative to whatever). What you
want to know is how to get the power back on. Your speed would usually
be of no consequence as long as you don't have power.

If you are headed for a rendezvous point, though, it might be crucial to
know your speed so that you will know when to use your battery-powered
short-range radio to let your friends know that you need help.

If you are headed for a star, knowing your speed will just tell you how much
time you have left before you die. Same if you are trying to get away from
Darth Vader.

Using a spectroscope to measure redshift and blueshift of stars around you
works down to a few kilometers per second, I think. Maybe less. It depends
on the quality of the optics. And it doesn't require knowing anything about
the stars beforehand.

-- Jeff, in Minneapolis

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http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

I prefer to toss something lightweight out the window, such as tufts of grass, or Schrodinger's cat, and see which way the wind takes it.

lol, I meant the "real sci-fi" as "hard sci-fi" as scifi that respects the natural laws

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They propably know where they left and/or where they're going. The rate of the change in distant to one or both of those points propably could give a hint. Those good folks steering the darn thing, propably know better ways to meazure the distance than I do