There are several things people can refer to when they speak of 'preferences'. First off, you may want to look at the descriptions of both INFJ and INFP to decide which one fits you the best. If you'd like to take a more general route, you can look around for differences in Judging vs. Perceiving, the two letters that represent how people prefer to 'manage their lives'. There's also something called a functional analysis test which you can take that's available in a sticky in the Personality Test Resources forum, which may be able to tell you which type you relate to more in a different way. Those are some of the preference methods - if you'd like a topic for outside evaluation of your type, I'd recommend starting a new topic in the 'What's my personality type?' forum.