What method do scientists use to tell how far a galaxy is?
If you know the brightness of a star and at what scale light reduces with distances we can calculate the distance,.Some variable stars have a relation between its luminosity and period.Example cephied delta. If you find a variable star like this in a galaxy we can use it as standard candle and calculate the distance. Type 1a supernova can also be used for this purpose. refer Wikipedia cosmic distance ladder.