How do you identify reducing / non-reducing sugar by looking at structure? Identifying reducing / non-reducing sugar been confusing me for a while now , I know that reducing sugar contain aldehyde or ketone group . It's easy to identify them in monosaccharides but this becomes confusing in case of disaccharides! 


I fail to see carbonyl group here, How can I identify reducing / non-reducing sugar by looking at structure in picture? can anyone help me out?
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.