Scientists use climate proxy records like coral skeletons, tree rings, glacial ice cores, and sediment layers. For example, the levels of oxygen 16 in a layer of ocean debris and fossils go up as temperatures rise. So a high level of oxygen 16 in sediment from one layer tells scientists that the planet was hot and watery when the sediment was laid down.
Good idea - just made a YSK post about it here
the_final_countdown.mp3