I think my boyfriend's been cheating...?

So before I start I have to tell you a little background, he and I live together. My boyfriend is big into card games and stuff. I don't really understand most the games he and his friends play. Sometimes they bet for money. Anyway there was this tournament thing coming up and he kept leaving to "practice" for it. I would often see another woman dropping him off and when I mention her he pretends I don't know what he's talking about, which I thought was really rude because he never invites her in or anything and I think it's a pretty long drive. So one day the woman comes and picks him up and I decide to follow them, I get on my bike (we can't afford a car okay) and try to keep up. If you've ever tried to follow a car on a bike you know it's no easy feat. I finally caught up with them, they were stopped at the beach. I saw a couple making out but it was a distance so it might not have been them, the woman's car was parked there though. By the time I caught up the couple was gone so I don't know who that was. The woman's car pulled away. I continued to the friend's house (where my boyfriend said he's been practicing for the tournament) and saw her car there. I went around and peeked in windows until I got to one with a good view. I looked in and there were four of them there playing some silly game acting all serious as usual (my boyfriend, the woman, the friend who's house they were at, and his roommate). I saw my boyfriend with his arm around the woman, and later I think I saw her kiss him, but my boyfriend said later, when I asked him about it, that it was just a trick of the light. Anyway, as I watched my boyfriend kept winning, winning and winning every game! Then I realized the deck he was using is one of ours and it has been marked so he can tell what the cards are! I think he is going to cheat on the tournament, should I call him out?