It’s been verified repeatedly.
Devices were left on the moon that are detectable by laser scatter, or something like that. I believe they are to precisely measure the distance from the Earth to the moon.
Anyone with the right equipment can check at any time.
No before anyone says it, no, NASA isn’t the one with the right equipment.
Well NASA has the right equipment. But they aren’t the only ones. You can actually get it all from Edmund Scientific. Last I looked NASA had a webpage that told you what to buy and where to aim your laser and how long it’ll take the laser to bounce back. It’s kind of fun.