How do I identify how many networks there are given an image?

So given an image, how do I know how many netoworks there are? here are two sample images:

http://imgur.com/YSYVH So the first one has 5 networks (says the answer) =\

http://imgur.com/zm5Ri and this one, the question was "Assume that all devices are using default settings. How many subnets are required to address the topology that is shown" and the answer is 1 subnet only... Is this because of the "default settings" part? or because it's only using switches? =\

We have a quiz coming up and I really want to be able to identify networks >_>