On the Gingerbread version of Google Talk, I can see a gray-ish silhouette of the Android logo on my contact. Now on ICS, I can't see it anymore, thus, prevents me from knowing if a certain person on my contact is on mobile or not. Is there a setting that I'm missing here? How do I know if my contact is on Android or not?
Essentially, you can't. This feature was removed from the 4.0 version of Google Talk, and is discussed somewhat in the Android Police review of the changes from 2.3 to 4.0. There's no setting that will bring back the icons.
However, when you have a chat open with someone (provided that they do not have a custom status message set) it will inform you at the top if they are on a mobile device:
My hunch would be that Google doesn't really want you to care about this when you're on your phone. They want the Google Talk app to be as seamless and integrated as possible, and they want you to use it regardless of what device the other person happens to be connected with at the time.