How to determine if he is a player?

Okay so im dating this guy that I have met before at basketball games, and when I was with friends and I never really thought about us dating, so one night his cousin randomly starts talkin to me on a social site, and he is like do you know my cousin I said yeah why and he is like well he really really likes you and thinks you are sooo beautiful. I said okayy well I have a boyfriend sorry, then he(the guy that likes me) (my boyfriend now) starts talking to me he asked me if I had a boyfriend and I told him yes and he goes well this sucks because I really like you and think that we could have a great relationship, and he goes ever since the first time I saw you I thought you were beautiful but you were dating one of my good friends, so I never really talked to you, and he is like are you going to the fair tomorrow I said yeah probably he goes well do you want to meet me there and we can hangout I said yeah sure.. well by this time I didnt have a boyfriend anymore and we hung out all day and went swimming and stuff like that and I liked him and he asked me out that night and I said yes, and most of my friends say he is a good guy then the others say he is a player and stuff like that and ever since I have been dating him a lot of people are starting rumors trying to break us up, he always says that he loves me and never wants to break up and he plans on marrying me, he asked me to marry him, and he is never mean, or anything we never fight, but it all seems to good to be true so I dont know what to do, and we kinda jumped into the relationship pretty fast and I told him I didnt know much about him and he said isnt that what dating is about getting to know eachother better. Just so you know we have been datin almost 3 weeks and im 14 will be 15 in october and he just turned 17 two days ago.