Good question. If you have to ask you probably don't have a clue anyway ;-)
Now, seriously, this has got to be the most loathed question on any Gothic-themed mailing list or forum. For the most part people come to these lists as a kind of sanctuary from this question, a place to converse with like-minded people who aready have a clue, and net.goths as a general rule can be pretty snarky on line. This is more of an in-person type question. Someone who might be inclined to bite your head off on-line over this faux-pas will quite likely be quite personable about it over a smoke outside the club or some such other IRL situation. The worst thing that is going to happen to anybody on-line is being banned from one forum or another, so people tend to be considerably less concerned about causing offence on line than they would be face to face.
If you are new to the scene, don't sweat what is or isn't goth. Just go with your gut and don't sweat the details.