To begin a flyover city tour, you must first be using Maps in Hybrid mode. If a city supports flyover tours, it will be labeled in yellow instead of white. Supported cities include: Rome, Stockholm, Barcelona, New York, Paris, Glasgow, Cape Town, Perth, Bordeaux and the Bay of San Francisco.
Once you click the yellow city label you will see the option for the flyover tour. The feature is currently available in the latest Yosemite and iOS 8 beta; however, currently you need to click the city to access it. Searching for cities will not give you the option. The fact that this UI is so hidden and no one has discovered it until now suggests that Apple will make some tweaks before iOS 8 is released to the public this fall.