My response, not to be factitious, would be - why are you worrying about how Noah knew it was God, when this happened so long ago, it is before the patriarchs? It happened! Noah knew! You are not Noah!
I think Noah had it confirmed he had heard from God, when it started to rain, and the fountains of the deep opened up. So would God call us to be Noah, and build an ark?
Well, God promised with the rainbow there would never be a world wide flood again. So if you try and apply what happened to Noah to your own life, you are going to be wrong. Now, if you live where I am, and my large lake is at an all time flood high, for the history of this lake, and getting higher, might be advisable to sand bag your property if you are near the lake or any of the affected creeks bringing down snow melt from the mountains.
We have to use wisdom. Not everything that happened in the past is going to happen to us. But, there are important lessons to be learned. What can we learn from Noah's example? Well, sometimes when we know God has told us something, we do need to do it, no matter how small, no matter what anyone says. And how can we live righteously, is another important lesson that comes out of Noah and the ark. God was displeased with the people, who were living evil lives. What pleased God about Noah and his family? What displeased God about the rest of the people? That is a whole area of study - how to live righteously before God. (Not talking "self" righteously, either!)
As for hearing the voice of God, maybe that is another thread?