This is a really great question. First, I would argue that it is actually easier in retrospect to see parallels and connections between subcultures than it is in the present, especially because subcultures aren't manufactured entities (or at least are never exclusively so); they are "cultures" in the sense that they are formed by a diversity of individuals who share some similar consciousness by participating in a specific event/activity (which makes them a sub-component of a larger culture). Paradoxically, it is often difficult to describe exactly what that activity is in total while it is ongoing (for instance, you might say you were listening to punk in 70s/80s Britain, but in retrospect you might refer to a constellation of activities and beliefs that constituted "being punk" in retrospect). OK FINE, but that wasn't what you asked, so...:

One way an historian might approach it would be to look for a subculture's emergence. That can be tricky, however, because you have to choose the element that supposedly makes punk "punk" and goth "goth". Often it is music (though I'm not sure there is a "metro" music scene -- perhaps Metric?)

Since at least the 70s there has been a really rich body of literature on cultural consumption, i.e., what products people buy to signify or express identity, how producers market and design products, and how these are shaped over time. For your goth example, you described fashion (buying clothes) and cultural consumption (going to shows). Both consumer might identify with Goth culture, and only really participate in one realm. But the realms are joined by the large overlap between the two, which makes Goth a real subculture, not simply just a style of music (or dress). The two are intimately linked, to be sure.

Historians might also focus on performance -- taking "culture" in the traditional sense of being related to art or creativity. Indeed, performances can be ways of synthesizing a subcultures generally shared views -- aesthetically, morally, ideologically, and so on. This approach might center on a particularly standout performer, artist, or creator. I personally don't know much about goth subculture, but I would associate Bauhaus and the Cure with it -- either of which might be used as a lens through which to look at goth culture's development.

You should check out Dick Hebdidge's book Subculture, which actually addresses goths (though it is mainly about punk) in 70s/80s Britain. There is extensive literature to review, but I can list some classics if you are interested, both displaying the sort of methods I've listed, and some theoretical/methodological texts.