How to differentiate the notation of a compounds charge and an anions oxidations state? I have had the case a few times. Being given something of this form:

$$\ce{Cr2O7^2-}$$

How do I know whether the $2-$ is referring to the oxidation of the oxygen anion or the charge of the whole compound?
make sure to read up about that topic. The concept of charge and oxidation state are closely related but aren't the same.

Usually oxidation state is written with roman numbers (that's how I learned it) and are not explicitely written like in the example you showed. 

Charges on the other hand are always written (if mentionned) on the upper right side of the anion.

I am far from being an expert, but thinks this might be an answer to your question.