How do I know if my boyfriend is cheating on me with my best friend?

He has been ignoring me all day today and half the day yesterday. But my best friend is sending me messages telling me that he's been talking to her all day. She is completely gorgeous and we are both site models. They just met yesterday but he's kind of a man . I just need to know how to tell if he's cheating on me with anyone...especially my best friend.