I can't wait to show you more and more how wonderful rats are @bitfiend. Depth perception- got it. That is great advice.
I haven't really taken drawing classes. There were some general art classes in school but I don't think I was present enough for it to count.
I found this(external link) to be a good read on understanding depth perception in a drawing but it is a little advanced. I was just reading around. It shows what I consider professional examples of depth perception but it let me understand it easily. Basically, the "looking down a long hallway" effect.