Having converted to Judaism, I’m a little out of practice, but at one time, I could make a fair guess about the religious affiliation of others based on dress and appearance – indeed, a friend of mine and I used to do it for sport when we were out at certain restaurants. The cues I used were no doubt somewhat regionally determined – I doubt I’d have done as well in the Upper Midwest, North, or West, but in Arkansas or in outlying suburbs of Atlanta it wasn’t too tough. Some groups (Assembly of God, Church of Christ, Nazarene, Pentecostal) can be fairly readily identified at any time, for reasons alluded to above: specifics of women’s hairstyles, types of dresses worn (no pants on women on scriptural grounds), etc. Baptists are a little less obvious, but can be readily distinguished from the former groups, and generally are somewhat different from Methodists, Presbyterians, and Episcopalians. On a Sunday, however, you can make even finer distinctions, being able to tell the Presbyterians from the Methodists from the Episcopalians fairly reliably.
Understand that aside from a few prescribed (or proscribed) modes of dress and appearance, these are mostly socio-economic distinctions, and have little or nothing to do with the tenets of a particular denomination. For most Southerners and Appalachians, particularly those in small towns, denominational affiliation is as much a matter of who your family is, whether you own property, and what you do for a living as it is about doctrinal differences. And these distinctions depend on being in a place that is large enough to support multiple congregations (the smallest communities often have only one church, whose denomination shifts to that of each new minister, the congregation having to take whomever they can find), but also small enough not to become “cosmopolitan”, where numerous other opportunities for group affiliation dilute the effect. Social scientists have frequently described what are called “group contrast effects”, where people who are divided up into initially arbitrary groups soon develop distinct identities, distinguishing themselves from other groups by emphasizing whatever differences they can come up with, however slight these difference may have originally been.
As for the relevance of Clarice Starling’s mental assessment of the woman, while it may do nothing to advance the plot, it does suggest things about her character. I’ve only seen the film version of Silence of the Lambs, having read neither of the novels, but as I recall Clarice is from small-town West Virginia (doesn’t Hannibal Lecter play on her insecurity about this at one point?). An intelligent, observant child in such circumstances is intensely aware of the finely distinguished social strata in the community; such awareness is a necessary precondition to defining oneself against those one wishes to leave behind. The Lutheran bit is a little surprising to me, as Lutherans who aren’t transplants are pretty thin on the ground in the parts of the South and Appalachia I’m familiar with (outside of a few areas settled primarily by German immigrant farmers, like around Stuttgart, Arkanssas). Is this explained or dealt with elsewhere?