The history of marbles can be traced back thousands of years, with early examples of the game being found in the graves of ancient Egyptian children. For many hundreds of years, until the early 19th century, these marbles consisted of polished stone -- ideally marble, hence the name -- or fired clay. In the 1800s crudely-patterned ceramic marbles were produced, but these waned in popularity once colourful glass alternatives started to be made on a commercial basis during the 1850s. These marbles, handmade by skilled European glassblowers, in turn gave way to machine-made American ones in World War I (WWI).
Look for any marbles that are not made of glass. Ceramic, clay or stone examples should be easy to distinguish from glass because they will be much plainer and less colourful. The stone and clay examples will have no decoration at all, while ceramic examples might have a dull floral print. All of these marbles are likely to be pre-1850.
Go through your glass marbles carefully, looking for "pontil" marks. These tiny wrinkles in the surface of the glass occur when an individual marble is sheared from a molten glass cane being handled by a glassblower. They are therefore a sign that the marble is handmade. If a marble has no pontil marks, then it is machine-made and this will date it to post WWI.
Look for any marbles with two pontil marks, one on either side. This tells you that the marble was made in a batch by a glassblower specialising in the manufacture of marbles. This indicates an approximate 1850 to 1914 date of manufacture.
Check to see if any marbles have just one pontil mark. This a sign of an "End of Day" marble. Before marbles were commercially manufactured, glassblowers used to make one-offs as presents for their children with whatever scraps of glass were left over from the day's work. These are likely to date from before 1850.