How would you find the density of a gallon of milk?

Density is the mass divided by the volume

Density can be worked out from the weight of a product. Density is mass divided by volume.

To do this you will need a container of which the weight is known, an accurate scale and 1 liter of milk (measured accurately). Weigh the milk and the container, subtract the weight of the container. The density should be roughly 1030 - 1036g/L.

Another option is to get a densitymeter with a scale of 1.000 to 1.100g/L. Put the densitymeter in the milk and read the density off the scale.