Quote:
Originally Posted by Jens
It's easy. You lick your finger, stick it out the window, and feel which way the wind is coming from.
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Not that far off really.
The best answer depends on the scale of speed that the ship is moving, and the extent to which "all instruments" are unavailable.
If, for some reason, the speed is non-relativistic, but still much higher than the speeds of our current space probes, you could use a spectrograph if you had it available. You could measure the pulses of pulsars if you had the right instrumentation. But, the OP said no instruments... That pulsar thing would be difficult even for McGuyver without instruments.
But you might be able to make a reasonable estimate of the speed relative to the interstellar media simply by noting the temperature of the outer hull on the leading side, vs, trailing side of the spacecraft.