Assuming that all their light is coming from stars, they have far too much stellar mass (with stellar masses similar to the Milky Way) to be the progenitors of today's globular clusters.
We have models for how much light a star of a given mass, age, chemical composition etc. puts out as a function of wavelength. We can then take mixtures of stars and predict the total light output of a stellar population in different bandpasses. So from observations of a galaxy in these different bandpasses, we can use these models to determine things like the age of the stars in the galaxy—from how old the stars in the best fit stellar population are—as well as the mass (or number) of stars—from how many are needed to match the observed brightness of the galaxy.