When it comes to heresy, doctrine that is damnable and not disputable, sometimes it can be easy to spot and sometimes it isn't, or in the very least, it's harder to mark a heretic as a heretic. There are a few examples to give here.
A Blatant example of heresy would be the Word of Faith movement. We get those who blatantly teach heresy, namely Joel Osteen, Benny Hinn, The Copelands, The Crouches (one of whom passed away last year) etc, but there are the more subtle ones like Joyce Meyers. She is subtle in the fact that many claim that her teaching has been very beneficial, but there are also many things that she has taught that are heretical, including her word faith preaching as well as the Jesus dying spiritually heresy. She is not as obvious to spot on the surface from what I have observed from others, but certainly her false teaching has been documented.
We also have this when it comes to Modalism, the subject of Itzhak Shapira. If anyone is familiar with what happened a few years ago, Sam Shamoun and Edward Dalcour both examined what Shapira had said in a video response to Yisroel Blumenthal.
You can Dalcour's comments here: http://answering-judaism.blogspot.co.uk/2014/02/video-examination-of-itzhak-shapiras.html
There are two types of Modalism that exist, Successive and Synonymous and they both teach Jesus was a manifestation of God, rather than Jesus being a distinct person from the Father and Vice versa.
Successive Modalism, teaches that the Father existed at one time, then the Son at another time and then the Spirit at another time and is an earlier form of Modalism.
Synonymous Modalism however teaches that all the three manifestations co-exist at the same time. In both cases, Jesus is all three manifestations simply taking different roles.
The latter is harder to spot and is funnily enough a common form of modalism that hides in plain sight and to the untrained eye, can easily disguise itself as Trinitarianism when it isn't and for the most part, it's a doctrine that is held to in ignorance.
Unlike all forms of modalism, The Trinity teaches in the very being or essence of God, there exist Three Distinct persons, Not three beings in one being or three persons in one person (like Ahmed Deedat falsely claimed). There are not three separate Gods, but one Eternal God which the TANAKH and the NT make very clear. The Father is not Jesus, Jesus is not the Holy Spirit and the Holy Spirit is not the Father. Modalism however does not make this distinction and treats all three persons as manifestations, they do not treat the members of the Godhead as divine persons. Again as said before, a synonymous is harder to spot than a successive modalist. Just asking them if they believe in the Trinity and pressing them will either reveal the person to be at best, ignorant and need education on the Trinity or at worst, believes in a false Christology and will not repent.
Universalism is also another example, there is Universalism that proclaims you don't need Jesus to be saved and all paths lead to God and there is Universalism that proclaims Jesus is the only way, but will be purified in hell and accept him as Lord when he comes to judge the quick and the dead.
Both are absurd and it's an obvious given that Unitarian Universalism is false but the other form of Universalism is harder to deal with, as the concept of hell isn't necessarily denied and possibly in some cases, one could be an annihilationist and be a universalist at the same time, it really depends on the individual who holds these views.
I have already spoken on Universalism in other papers, namely some of the biblical references used as well as looking at some of the church fathers on the subject to see whether or not they taught it:
The latter form of Universalism seems to allow for one to proclaim that Jesus is the only way to heaven while adding the idea that all will be saved by Christ's death after a time in hell and that Jesus brings unbelievers out of hell later. It's just as deplorable as Unitarian Universalism while at the same time trying to shoehorn itself into Biblical Christianity. Satan never tells an outright lie, He mixes truth with lies. This is why sometimes people will inadvertently accept people who may have a false heretical belief. We must be on our guard against heretical beliefs.
The same scenario for the other groups I mentioned also apply to Roman Catholics and Mormons, especially when they claim faith in Jesus or claim that they themselves believe in God's grace, when the truth is they mean something different from what Biblical Christianity means by grace. A definition of certain terms is important to clarify, as your definition of grace may not be the same as theirs.
As Jacob Prasch once observed, people have no problem calling Charles Taze Russell or Joseph Smith false prophets, but an unjust balance is an abomination to the Lord and the scriptures themselves make clear we must not have uneven scales and unequal measures, for the Lord detests them both (Proverbs 20:10, 20:23). I say this because of the Kansas City Prophets, which would include Paul Cain, Rick Joyner, Mike Bickle, Lou Engle, the late John Paul Jackson, Cindy Jacobs and others. We are so quick to point out the false prophets that lead the Jehovah's Witnesses, Mormons and Islam, yet fail to overlook the numerous false proclamations of the Kansas City Prophets. Why do we examine one, but not the other?
Deuteronomy 18:20, despite it's dismissal by many still holds weight today that false prophets are to be rejected should they even have ONE false prophecy.
While under the New Covenant we are not to stone false prophets as found in Deuteronomy 18:20, this doesn't downplay the seriousness of false prophecy and the person who made the prediction should be rebuked, brought to repentance and must cease involvement in prophetic ministry. Contrary to the claims of The KC prophets, JWs and Mormons, One prophecy is enough to disqualify you for being in that office.
Heresy can be blatantly seen or in some cases, it can be well hidden and needs to be investigated to see it for what it is, but it doesn't change the fact that both are equally as dangerous. Guard yourselves lest you fall.