In the following mechanism, nitric acid acts as an oxidising agent to remove 2 H atoms from the intermediate (in the blue square)
My first instinct if given only the reagents would be to protonate something, such as the amine group for example, rather than removing hydrogens.
So my question is how can I identify if $\ce{HNO3}$ will act as an acid or an oxidising agent, given only the reagents? Do I just have to try out both and see which one leads to a reaction? Or is there a property of nitric acid that I'm missing?