I think my wife maybe cheating on me with my best friend how do I know for sure?

He is over with her while I am at work and always buys her gifts, she said's there is nothing going on.

if he is truly your best friend you should ask him and see. if he tells the truth, he is a true best friend, if he dosnt, why you even calling him a friend, let alone, "best" friend.

Ask your boss to let you go home for a few minutes during the day when you know your "best friend" will be there.

Do you know your wife? Her behaviour should tell you. She will spend more time on herself when she knows he is coming over, she will try and avoid being intimate with you, etc. DO you know your wife or are you just jealous?

fem said it all !

Dude. You seriously need to nip that relationship in the bud. Not to alarm you but that's how affairs start - or continue. For the most part guys and girls can't really be just friends. The line usually gets crossed at some point. I think you have a problem on your hands. Good luck.