One way to determine when glacial periods existed in the past is to look at fossils and determine the ratio of Oxygen-16 and Oxygen-18. Oxygen-18 is an isotope (type) of Oxygen with 2 more neutrons in the nucleus of the Oxygen atom. We know the current ratio and when a glacial period occurs the more common Oxygen-16 gets trapped in the ice leaving the oceans Oxygen-18 enriched. This can be measured in the fossils from sea creatures and when these fossils are dated we learn when glacial periods existed in the past.

(The Great Courses – A New History of Life)