Wrap a cloth measuring tape around your head, resting it where you like to wear your hats, and jot down the measurement. Then head to the retailer's website to determine which cap size corresponds with your head's measurement.
One of the most venerable and popular hat manufacturers, Stetson, suggests that if you are between two sizes, you should select the larger size. Hat pads are available to tighten the hat's fit, if necessary.
Village Hat Shop offers a second option for ascertaining cap size. Take a string, and place it around your head, about an eighth of an inch above your ears, holding it securely but not tightly. Mark or clip the string, and then measure it against a measuring stick or ruler. Like Stetson, Village Hat Shop says that for in-between sizes, go with the larger size.
There is one other means for measuring cap size: find a hat that fits you well, and check its size.
Hat sizes are universal, but they still vary somewhat between manufacturers. It is always best to try on a hat before purchasing it, just to ensure correct sizing. When this is not possible, measuring your head and finding the corresponding size should get you a cap that fits well.