For new Sones distinguishing who is who in Girls generation can be hard.
But after a while it slowly becomes easier.
One of the ways I found really good was to watch their shows.
Here is one of the first Episodes of one of their show and another good one is The horror movie factory.

Here are also some images I got off another website ( I did not make these)http://super-kpop.blogspot.com.au/2009/07/how-to-differentiate-snsd-members.html