system
Given the following post, respond as if you're commenting on Reddit:
human
How do we differentiate redshifted light versus red light?

Hey, thanks for the explanations in advance! 

So I believe I understand the mechanics of redshifting. The waves  stretch due to the acceleration/movement away from us. My question and lack of understanding is how we interpret and differentiate the redshifts.

Using completely arbitrary numbers for simplicities sake; assume we have a lightsource that emits light with a wavelength of 5 that we see with a wavelength of 10 due to redshifting. How do we differentiate that lightsource from another lightsource that emits light with a wavelength of 9 that redshifts less and we see it as a wavelength of 10. 

Is there some other quality of the light that gives us hints as to its original wavelength, or are we just making educated assumptions based on other factors?

Thank you again.
gpt
Different elements have different emission spectrum. So for light from stars we can see, say the hydrogen lines, have been red (or blue) shifted because we know where they are on the spectrum when the source isn't moving relative to the observer.