POD NyaChan: Enough married people are monogamous that outside a very few contexts (and an apartment complex is not such a context), it's not prudent or kind to assume otherwise. You can ask, politely, but only if you're prepared for a response along the lines "How can you even think that?!
The neighbor here didn't send a note saying something like "I know you're married, but if it's open, I'm interested" or ask directly "so, you're married, does that mean you're monogamous?" Nor did she wait for OP or her husband to say something (in the universe next door where they have that kind of relationship, and if he was interested, he could send a note inviting her to meet them for a cup of coffee, and maybe take it from there). But there seem to be a lot of single people who will happily make passes at married people, and back off if the answer is "sure, but I have to talk to my wife/husband about it."