fontspec, and XeTeX or LuaTeX it's possible to access a font's OpenType features, including the Style Sets, e.g. like this:
TeX.sx has already taught me how to find out which features are available. Assuming I don't have proper documentation of my font at hand, is there a way to find out which glyphs (in which contexts) will actually come out differently when I use a certain style set (or another OpenType feature)? It also seems to depend on the script and font family (e.g. small caps). I feel like LuaTeX would be able to do this.