Symbols and images.
The past: recent (usually, a reference to the previous scene), distant past, memories.
The future: soon (usually a reference to the next scene), events to come.
Characters: Internal - Attitude. Relationships. Feelings. Concerns. Wants. External - clothes, hair, shoes, props.
Dialogue: Subjects. Things not mentioned (ie subtext).
Information (ie things the reader needs to know).
Actions: What happens?
Scene ending (ie does it make reader want to read next scene?)
I go through the list checking that I've included every element in my scene. It sounds a bit mechanical but it's a good way of making sure that you're covering the scene properly.