First, these remarks refer to self-identified Christians - people and churches who explicitly claim to be Christian, and explicitly regard themselves as Christian.
In this situation, the
proper framing of the question is to ask whether some person or church who says they are
Christian - and shows all signs of sincerely believing that they are
Christian - really is what they profess to be and what they show signs
of believing they are.
(When I say 'sincerely believing' I mean that we need to judge that a self-identified Christian person or church is not claiming to be Christian merely in order to attain some other non-Christian goal; an exclusion which would apply to many modern Leftist Liberal Christians - in fact a majority of the leadership in mainstream Christian churches.)
When a person (or church) says he is
Christian and we believe that this is sincere because he also behaves as if he believes that Jesus is his Lord and
Saviour; then the principle is to approach the claim with charity, and
with a genuine hope that the claim is true.
Then, observe the situation over time, check what is happening to that person or church.
example of a borderline but false claim of being Christian was
Unitarianism - which seemed, initially, just like Protestant
Christianity (from which it evolved) and involving the same personnel - but denying the divinity of Jesus. It was not completely clear at first, because the earliest Unitarians included people who seemed sincere and
devout and well behaved - and were clever enough with words to seem to be able to explain that they still were Christians...
Denying the divinity of Christ would
seem to be a clearly non-Christian belief anyway (although many Anglican
Bishops and theologians would perhaps disagree) - but if the fact was not at first obvious, it soon became obvious by trajectory into apostasy of many well known
Unitarians (such as Ralph Waldo Emerson) and of the Unitarian church
itself - and this just increased with every decade.
An example of the opposite situation is the Monophysite controversy which tore apart early Christendom.
This vicious heresy war was concerning what - in retrospect - was a mere philosophical quibble: a distinction without a difference. We know this for sure because the Monophysite church has continued for more than 1000 years to be self-professing and sincere Christians - who have show no tendency to collapse into apostasy.
The people who regarded Monophysites as non-Christians turned out to be just plain wrong.
The same applies to the Church of Jesus Christ of Latter Day Saints, although the difference from preceding Christianity was much larger.
Mormons have always professed to be Christian, and shown all the signs of being sincere and devout in this profession.
Initially, it was understandable that mainstream Christians might regard Mormons as non-Christian, and this belief was assisted by the truly epic scale of mass media misrepresentation and just-plain-lying concerning their reporting of the early polygamous phase of Mormonism and of life in Utah.
But over the space of 184 years the CJCLDS shown no trend towards sliding into apostasy - therefore, according to the criteria I have outlined, and from any charitable perspective: Mormons, like Monophysites, just are Christians.