Testing of isopropyl alcohol percentage using graduated cylinder and scale

Bought a gallon of 75% isopropyl alcohol (rubbing alcohol) for about US$12. Some friends feel this is "too cheap, it must be fake or watered down", so I decided to find out.
I'm going to assume the contents are only isopropyl alcohol and water, as it would not really be cost effective to water down with something other than water.
I only had a graduated cylinder with 5mL markings, so there is certainly room for improvement on the accuracy.