The Oakland Raiders "did a lot of work" investigating Gareon Conley before selecting him in the 1st round of the NFL Draft and believe he's NOT a rapist ... so says Ian Rapoport.

The NFL Network insider -- one of the best football reporters in the game -- says the Raiders dedicated serious resources to finding out as much as they could about Conley's rape case before they selected.

Conley insists he's been falsely accused of raping a woman at a Cleveland hotel on April 9. He has not been arrested or charged with a crime.

Get this, Rapoport says there was also interest from other teams -- Conley even took a lie detector test to prove he's telling the truth.

Bottom line, Ian says the Raiders were a solid team before the draft and if the Conley pick works out ... they're gonna be scary good.