The maker's mark is the easiest way. Usually, the marks are embossed at the base of the vessel along with other letters and/or numbers, which generally refer to mold numbers/forms. If the mark has an...
James St. James is 45 years old (birthdate: August 1, 1966).
James is 4 years old (birthdate: December 17, 2007).
Uh... twice David's age?
Check for jam jar labels in the canning section of any hardware or grocery store.