Usually the tool you'd use for that is a human being. I'd be pretty shocked if there were tools like that for any language that actually worked accurately. Although, I'm pretty sure I could detect for(){for(){}} and produce O(N^2).

It's not an exact science in any case. A lot of the cooler parts of the proofs tend to be knowing which parts of the math don't matter (and showing it). That's more like art. Mathematical art, but art. I dare a computer program to do it.

You might be able to get something to measure time slices for given inputs or something, ... I'm not sure that would really show big oh though.