How do you identify reducing / non-reducing sugar by looking at structure?
Many sugars are drawn in the cyclic, closed form where the carbonyl group has been converted to a hemiacetal.  



Once you realize that a hemiacetal can equilibrate with a carbonyl (e.g. it is a carbonyl in disguise), identification of reducing sugars becomes easier.

Here is an example from Wikipedia, it is Maltose, the same as your third compound



See the aldehyde in the open isomer drawn on the right; see how it can close to form a hemiacetal on the left.