Actually this is a really good question. If we look at some light of, say 500nm wavelength, how can we tell if it was originally 500nm or if it has been redshifted from, say, 400nm. In this scenario, there wouldn't be a way to tell. However joinedup is correct in that, if we know what we are looking at elementally speaking, we can work it out. You'll recall that every element has a unique 'fingerprint' in it's line spectra, and so if we know we are looking at light from a cloud of hydrogen, where we expect spectral lines at (and this is made up...) 410 and 550 nm, but we see spectral lines at 480 and 620nm, then we know it has been redshifted.
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