Your best bet is to check out the admissions information schools put online, and carefully look at the student profile they present, as well as any student highlights on their webpage. Of course, you should feel free to email them as well, as noted earlier.
My impressions of US-based IR programs is rooted in having witnessed the process my then-significant other experienced a while back, although she was interested less in academia and more so in non-profits/NGOs (which is where she ended up and is to this day). If you already have some economics coursework from your previous mathematics education, that is certainly a plus (if not essentially required by some programs) as well as any language skills you possess. The breadth of people that IR programs admit is pretty broad, so you aren't automatically excluded just based on the content of your undergraduate degree. Generally, though, if you can obtain relevant work experience of some sort - which doesn't necessarily have to be international in scope - it's a plus. Also, it will depend from school to school, but some will require a master's degree (or expect that you are in the midst of completing one) when you apply.