I acutally considered this, using shadows in such a way. I've seen shadows in both lucid and non lucids. The idea ocurred to me in a lucid where I could see my shadow on the ground before me. I wondered if it was laying properly in respect to where the sun was. I tend to notice light and shadow alot so a shadow reality check might work if I can remember to do it.

Like Mes mentioned though, something like that would take a lot of thought and the other reality checks are much easier. I'll give the shadow RC a go if I can remember to, just to check it out.