Well, there are three major yardsticks that we use to measure these things. First, there is the question of canon, that is, the Scriptures we use. Just which books are in the canon of the Bible is not as simple a question as it looks like, but one characteristic of groups that have left the central path of Christianity is that they have adopted additional authoritative books from which they derive their teaching. The LDS Church’s Book of Mormon and other documents fit this criterion, as do the writings of Mary Baker Eddy used by the Christian Scientists. New revelations typically bring in teachings from other, non-Christian belief systems.
Second, there is the question of Christology. "What think ye of Christ?" is the historic question that defines who is, and who is not, within the mainstream of Christianity. Orthodox Christianity is more or less defined by the Nicene Creed (No. 880 in our hymnal). This describes who God is, and within that definition who Jesus Christ is and what he has done for us. Even Churches which refuse creedal statements (such as those who say, "No creed but Christ") usually wind up describing God and Christ in terms that fit within the language of the Nicene Creed. The most common deviation from Nicene Christianity is Arianism, a belief that Christ is not fully God, which is accepted doctrine among Jehovah’s Witnesses and Unitarians. It is also common as a personal opinion of many members of obviously orthodox bodies (and, I am ashamed to say, of many clergy in those bodies). Bodies whose only major deviation from standard Christianity is doctrinal are often called "sects." In the case of Mormonism, their theology is definitely non-Nicene; indeed, they themselves admit it is polytheistic.
Finally, there is the matter of Church practice. Sacraments such as baptism and holy communion are almost universal among Christians (Quakers and the Salvation Army are notable exceptions). Living an ordered life which recognizes the special call and responsibilities of the clergy is also standard (again, there are exceptions). Mormons have sacraments and they look like the ones we use, more or less. What they mean by them is another matter. General Conference stated that, in our opinion (2008 Book of Resolutions Para. 3149), LDS baptism is not Christian baptism; therefore, we are normally to baptize a candidate for membership who comes to us from the LDS Church.
All this doesn’t mean that Mormons (or adherents of other, non-standard or non-Christian religious bodies) aren’t generally fine folks and good neighbors and moral as can be. But calling everybody you like "Christian" only means that the word loses all meaning. It also does not imply that some specific person will, or won’t, be saved. After all, there are lots of individuals in obviously Christian bodies who don’t believe their own Church’s teaching (we call these people "heretics"); likewise, there is no reason not to assume that there would be the occasional Mormon, or Muslim, or whatever, who believes in Jesus Christ in his heart just like we say everybody should (some call these people "anonymous Christians"). The Church exists to help you on your way to heaven, but just sitting in the train station doesn’t mean you will reach your assumed destination. That’s between you and God.