This table is a reference tool for rapidly locating Wikipedia articles on Historic and Prehistoric climate indicators of all types.
The most important aspect of this question is the word "prehistoric," which doesn't offer a very clear time frame for anyone to answer.
What I can say is that there are many ways to figure out what Earth's climate may have been like in the past. The scientists who study Earth's past climate are called paleoclimatologists - these scientists study things like ice cores, marine sediment cores, tree rings, speleothems, lake cores, and more.
An umbrella answer to this question is as follows:
Earth's climate has always been in a state of flux. Over the course of Earth's history, the climate has oscillated between periods of extreme heat and extreme cold, events which have shaped current land forms, ocean circulation, global weather patterns, and life as we know it.