system
Given the following post, respond as if you're commenting on Reddit:
human
How can you differentiate the influence of redshift from the doppler effect in an expanding universe?

So I learned that the redshift in the light from other galaxies is not due to relative movement, but because space itself expands in the lights way and therefore stretching the wavelength.

How can you tell, that the source of the signal is maybe closer than you think and just traveling away from us, therefore having a positive influence on the redshift of the signal

OR

The source is traveling towards us, therefore negating a little of the redshift (expanding space) by blue shifting (due to the doppler effect)?

I hope I made my question clear, however feel free to follow up.
gpt
They're in general impossible to separate. If there is cosmological redshift, then there is significant spacetime curvature between you and the source. And if there is, there is no canonical way of extending your coordinate system far from you, through that curved region, up to the source. (All the stuff with the clocks and rulers to build inertial frames makes no sense in curved spacetime). 

Since the only velocity of the source you can define is just the derivative of space coordinate wrt time coordinate, and the coordinates are essentially arbitrary, the velocity is arbitrary.

Playing around with coordinate changes you can swap bits of redshift between gravitational/cosmological and doppler.