I think part of what you are asking is, "If we can't resolve the disk of a object, how do we know if it is a galaxy or a star?" The answer is that we can tell a

I think part of what you are asking is, "If we can't resolve the disk of a object, how do we know if it is a galaxy or a star?" The answer is that we can tell a lot about a point source based on its spectrum. For example, quasars were first noticed solely by their spectra. The other thing to consider with distant galaxies is that they are moving away from us very quickly. The further away they are, the faster they are receding, and the more red-shifted their spectrum becomes. This is why JWST has different spectral response from Hubble.