Since the central characteristic of neurons is neurotransmission, the most obvious thing to do would be an assay that demonstrates stimulated release of synaptic vesicles. You could, for example, use FM-dyes or synaptopHluorin transfections and then stimulate, with electrical fields or high potassium, to see unloading of FM-dye or changes in fluorescence of pHluorin.

Can you do some quick IF assay use NeuN antibody?

The most widely used method to detect neuronal identity is immunostainning with Tuj1 antibody (against neuron specific BIII tubulin). It will give you an idea of neuronal identity but for testing functionality you'll have to use other methods as mentioned in the first answer.

Cheers