Standards of dress (at least for women) aren't nearly as consistent within Orthodoxy as we might wish. Wearing exactly the same halachically-correct clothing I have been under-dressed in some places and over-dressed in others, because I didn't necessarily know how minhag Young Israel is different from minhag Modern Orthodox or minhag Chabad or the others. For any particular community I can take a cue from others -- if in fact other women are present, not always a safe bet -- so I can get it right the second time, but is there some set of clues I can use to get it right the first time? And how do the expectations change for Shabbat services versus Shabbat-afternoon shiur versus weeknight shiur?
If I am going there in response to an invitation of some sort (e.g. a bar mitzvah) then clearly I can ask the people who invited me for guidance. I'm trying to navigate the challenges of wanting to go to an interesting-sounding shiur or to daven at a place I've never been before where I don't necessarily know anybody, without feeling uncomfortable (or making anybody else uncomfortable) the whole time I'm there because I guessed wrong.