How can the rest-frame spectrum of redshifted light be determined?

Question

The redshift effect must work on all frequencies, so blue would be shifted down to some lower frequency, and ultra violet would be shifted down also, into the visible area. How would you know what part of the (pre-redshifted) spectrum you were looking at?

Answer 1

Specific wavelengths in a spectrum, whether later shifted or not, are identified by the absorption bands of common elements near the emitters. For example, the spectral absorption lines of hydrogen are well known, and there is pretty much always some hydrogen near where the light was emitted. The absorption lines are shifted, but the various lines relative to each other are still a identifiable signature. It is by looking for these characteristic absorption lines and seeing how much they are shifted that we determine the magnitude of the shift in the first place, most of the time.