Can I buy a domain name if it is already taken ?

I want to buy a website name but the problem is that it is already taken. Would there be any way for me to buy the website? How would I know when it ends? The thing is my ex bought a domain name that I was going to use as my own. We started the site and came up with ideas about it. Then we broke up, he still has the website, and is being a jerk and won’t give it to me. He says he owns it until 2015, I have no idea if this is true or not. How will I be able to find out if it is really taken?