How can I tell the difference between a surreal and an expressionistic painting?

Thats kinda tricky,

Surrealism is a distorted reality for no obvious reason at all, an exploration into the mind of the maker.

Expressionism is a distorted reality that tries to invoke an emotional response from the viewer.

In a surreal painting the laws of reality are not obeyed yet objects are given realistic appearance though not necessarily as obvious as they first appear because some objects have two or more visual meanings or purposes, and can look two ways at the same time. Like the guy said "this is kind of tricky".

In expressionism the paint is used to evoke a feeling from the viewer. The object or scene will be secondary to the way it is painted. In other words, the way something is lite or the way the paint itself is applied causes an effect on the viewer because of it and not what is shown.

I admit, I'm more of an expressionist painter. I like paint and how its used in the painting process. Surrealism is great to but you have to be able to paint reality before you can create and break the laws of rendering an object as it is and I don't paint reality all that well. You have to be able to convince someone of the real nature of something in order to show the non-reality of that same thing.

The two styles are seldom used together. So if you feel that the painting is using more of a "painterly approach" then in all likelihood it is probably going to be an expressionist painting, if it is "kind of real looking but not" than it's most likely surrealism.