Whether you are talking about monotheism, salvation by faith alone, and other contrasting doctrines to Mormonism; Mormon missionaries may say, "Oh, we believe that too." It may not be inaccurate to posit the fact that postmodernism has seeped into the fabric of Mormonism. As a result, you may not hear a statement of, "our church is right and your's is wrong." But this is exactly what you want them to say and agree with. Such will allow for everyone's cards to be on the table.
With that said, Keith Walker has posted a very helpful ARTICLE. The article displays a list of quotations from Mormon sources whereby Christianity and Mormonism are starkly contrasted. Here is what I would recommend doing the next time you engage a Mormon; start the conversation off by showing them these quotations and getting them to admit that, for example, "all of [your] creeds are an abomination, and [you] are corrupt." This way, you'll be able to ask the question, "Please show me which beliefs I have that are an abomination and let's see what the Scriptures say about them."
Many of these Mormon sources are easily obtainable. It would prove useful to have them ready whenever a Mormon missionary knocks on your door. It will allow you to cut right to the chase and perhaps open the door to present the gospel.