Corn flour is the same thing as cornstarch. It does not go bad unless exposed to water or insects.

How can you tell if corn flour has gone bad?

As mentioned before, cornstarch doesn't go bad and doesn't lose potency with time. The only “normal” situations when you need to throw it out is when water or bugs get inside the package. Other than that, it will be fine. So if you open the package and there's mold or any other organic growth inside, discard it.

Is it OK to use expired corn flour?

Cornstarch is good to use in recipes indefinitely ([AS]). Unlike baking powder, it doesn't lose its potency over time. In other words, if you wanted to know if cornstarch (or corn starch) does lose its effectiveness, the answer is no. That also means that you can freely use “expired” cornstarch.