A personal perspective--I don't believe in gaydar. I don't think anyone can "tell" unless the person in question is signaling it somehow in their behavior or appearance, and even then, it can be prone to misinterpretation. I've heard people say, "Oh, I can always tell right away," and I think they're full of balderdash. When the display is particularly demonstrative (i.e., when the individual clearly wants people to know), that's one thing; when the individual is more or less "disguising" it (or simply not announcing it), that's very much something else.
In Ari's case, her difficulty in identifying others' proclivities can also be attributed to:
a) inexperience (she's only been "out" for a short while);
c) looking for love in all the wrong places. One would assume there are Lesbian bars in Portstown, but Ari likes hanging out at Sherry's, and as far as we know, she's the only Lesbian in the house.