Tim Tebow's former Florida teammate took drastic measures to confirm he was really saving himself for marriage ... 'cause Brandon Spikes claims he would randomly show the quarterback pictures of naked women to see how he'd respond.
Spikes -- who played alongside the Heisman winner in Gainesville from 2006-09 -- went in depth about the media obsession with Tebow's virginity during the Gators' dominance in the mid-2000s ... and he revealed he did some questionable investigative reporting of his own to prove the guy was, in fact, sex-free.