Mike,
I am not the judge of people... God takes care of this. God has defined sin and has given it to us via His law (we have it). Under the New Covenant, we now have His law and His Spirit within us that we may be directed according to His will. His will for us is nothing more than obedience to His law. The Spirit can not and will not lead you into anything contrary to His nature which is absolute holiness. You do have a will to go the other direction.
Under the New Covenant, our righteousness exceeds that of the scribes and the Pharisees. In addition, the Spirit within provides the believer a means for discerning spiritual truth (a provision provided to His elect under the terms of the Covenant). Take a look at 1 Corinthians 2:14.
The idea of discernment is that of "examination" not judgment. We examine all aspects of our life as Christians. We examine ourselves, our children, our relatives and so on. The church needs to examine itself sometimes.
Not only can Christians spiritually examine the state of themselves and those around them, but some are given the gift of discerning actual spirits (1 Corinthians 12:10). Discerning spirits amongst us, in others, in geographic areas, etc. If you have a topical bible, take a look at all the spirits that are listed (diverse, perversion, haughtiness, drunkenness, lasciviousness, deceitful, lying... the list is quite extensive).
The author of Hebrews is getting on to people in chapter 5 and 6 concerning their lack of knowledge. At a time when we ought to be teachers, we have become such that we need milk and not strong meat (Chapter 5:11-14). Knowledge of holiness has gone down the toilet for many. Knowledgeable Christians, those who can digest the meat and have moved on, leaving the basic principles, not laying again the foundation have by reason the ability to discern both good and evil (Heb 5:14).
So, to answer your question... yes. I can and most Christians can discern what manner of spirit is working on an individual based on two things:
1. What flows from their mouth.
2. Personal appearance (the outside of the cup).
This is all man has been provided to discern one's heart. Although you can not know for certain (because only God can see the heart), you have the ability to discern good and evil, light and darkness, righteousness and unrighteousness.
This concept does not only apply to Goth, but is used everyday all around us. It's spiritual warfare. If the devils can get you to desire a fashion of darkness... it is one step closer to bringing you further away from God. Take another example. If the devils can get you to watch p0rnography (hetero, bisexual or homosexual), you are one step closer to separating yourself from God. These are your gates (your senses) which you have a responsibility to guard against all things that are unholy.
Again, I am not the judge. All I can see is the outside of the cup. If it looks like a duck... it´s probably a duck. I am not sure why a "Christian" would want to identify with a sub-culture of darkness (whether it leads to the occult or not). Again the Spirit will not lead a believer into anything but absolute holiness. There is no communion between light and darkness. Goth fashion portrays an identity to a sub-culture of darkness (this is what the world sees). Christian Goth is a false witness.
... except for the piercings.
