If only there were an easy way to find out.
I honestly think people are making too big a deal about this. The kind of people who download apps are not going to have an issue exploring the UI of new apps to figure out how they work. And it's not that hard to figure out just by looking either. Verbs, arrows, and nouns that don't label something adjacent to them should all be clickable. And if they ain't, oh man, you just wasted two seconds finding that out. Not the end of the world.
As an example, the new flat google maps app is a disaster. It took me weeks to figure out how to do fairly simple things, like how to cancel turn by turn directions. My wife is totally lost, as a long time iphone 4 user trying to use the flat iphone 5 version. When she tries to navigate while I am driving, she is constantly confused while I say, "no! just swipe down hard enough that the app thinks you are going back, then click on the box, then click the x, then swipe up, and stand on your left foot!"
But we have to have a map, so we use it.
As a startup, this is what you should think about: Is your app as useful as google maps? If not, then you can't afford to make it unusable with a trendy flat interface.