If by "God", you mean depicting "supernatural" behaviour, then Krishna showed his universal form mainly twice. Both of them were before the Mahabharata war.
Once during the final peace negotiation (Udyoga Parva) and then during the discourse of Bhagavad Gita (Chapter 11). Refer:
How is 'Vishwa Virat Swaroop' of Lord Krishna described in Mahabharata?
In fact, both the times only few could bear the light of his form and recognise him as part of supreme.
After that he never showed such form. Regardless of that, he was never behind proving himself to be a "God". It were others who identified that.