Personally, I think they are quite similar to us in terms of their subconscious. We simply have had a greater range of experiece in our lives, and therefore are probably capable of imagining more horrific things.....

Man, it's hard for me to elaborate more on what I mean than that...

Anyways, as for how to tell.... I think you may just have to guess. Look for what you'd see in a human who's having a bad dream!