By far, the most effective Christian heresy hunting tool is finding any kind of relation between someone's belief and Gnosticism. This trick is used by both sides to literally demonize one's beliefs without charity or, in many cases, a single moment of investigation.
The problem is you can relate pretty much any Gospel-preaching, Bible-teaching Christian sect to Gnosticism because of the leech-like belief system of the Gnostics. They have borrowed and distorted from all of Judeo-Christianity, and its roots predate Christ's birth (a fact that seems to escape many). It's important to know what Gnosticism is to distinguish what to guard against and what to recognize as cheap strawmen arguments.
Basic Gnosticism goes something like this: The earth and all of matter was not created by the One True and Living God, but a "lesser" malevolent God which, depending on which era of Gnosticism you are researching, is either the Devil or a Devil-like being. Because of this, all matter -- including our own physical bodies -- are evil. Ultimately, all physical things must be denied to achieve enlightenment and avoid eternal damnation on Judgment Day.
Any student of Greek history will note this is a very Greek philosophy, and not one necessarily originating from a Judeo-Christian ethic, per se. In Genesis, the True and Living God created man and woman in the flesh and declared them fit for fellowship with him in their physical form. This is why Gnosticism generally repudiates the entire OT as heresy and the God of the OT as the malevolent one. Post-Christ Gnostics embrace the Gospel of John as divine, ironically in spite of the book's very deliberate relationship to Genesis. Also, those Gnostics did not believe Jesus ever took a fleshly form, and was instead a pure spirit who only looked like a fleshly form.
This is the Gnosticism Paul was fighting in Colossians. As you can see, it places a legalism onto salvation that denies the freedom of salvation in Christ. It creates a performance- or achievement-based element of responsibility on the person. It denies the physical suffering Christ accepted as part of the atonement for our sins. As Paul so eloquently counters in Col. 2:9, "For in Christ all the fullness of the Deity lives in bodily form"
Another more fundamental element to Gnosticism is the notion its followers possess some super-secret knowledge or understanding of God not known to the average human being. So Gnosticism put a high priority on the experiential and a low priority -- better put, a deprioritization -- of religious litany. The written word was almost impossible to reconcile as divine because it was of this earth, according the Gnostics.
The picture we see here is one where liberals can attack fundamentals of Gnosticism by any embrace of legal nature: i.e. total depravity, literal interpretation of the Bible, literal adherence of the Ten Commandments, literal anything. What is ironic here is those charging legalism are more likely to be in line with the Gnostics, because the Gnostics rejected the authority of the Apostles of the Early Church, rejected correction, and rejected things (such as OT as divinely inspired) that Jesus most definitely endorsed. This is not to say conservative Evangelicals are not capable of legalism, but legalism by itself does not equate to Gnosticism. Furthermore, both Gnostics and liberals have a lesser Christology than conservatives. Paul elevated Christ as the only path to heaven, and it is the conservatives, not the liberals, who uphold that view.
On the other hand, Charismatics and Pentecostals -- two groups who are sometimes at odds with fundamental cessationists -- are cited by some fundamentalists for delving into Gnosticism because of the emphasis on experience. While I have no doubt there are a handful in this camp that could be classified as Gnostics (I'm thinking of the kind that equate their own personal "revelation" as equal to or greater than God's Word), the great whole of Charismatics and Pentecostals still affirm all the fundamental tenents of Classical Christianity. It is not enough -- not by far -- to equate the Christian experiential with Gnosticism, and to do so shows an ignorance of both.
Where we find Gnosticism most clearly stated today is not in Christianity at all (unless you still insist on allow Universalists wear that title), but in the first world popular culture. It is in broad ecumenism that embraces generalities, such as "We are God" or "God is the God within us" or "God is our inner spark" or "inner flame" or "God is the elements of nature that bind us together." It is in pop-religions such as New Ageism, Kabbalah and neo-Buddhism.
Furthermore, heresy does not begin and end with Gnosticism. There are plenty other Christ-denying beliefs out there that do not engage a Gnostic system. Scientology, for example, very clearly has a system that denies the divinity of Christ and Classical Christianity, but is neither compatible with universalism (in spite of their claims) nor does it treat the physical world in the same harsh philosophical way (although there are many similarities that could lead one to believe L. Ron Hubbard copied freely from the Gnostics).