This may seem like a loaded question, for the word "realistic" completely undermines the entire genre fiction category, from fantasy to horror to science fiction.
But I don't mean realistic in terms of "Could this really happen?" What I mean is this: could the events be realistic in the context of the story?
In my novel Beholder's Eye, which I am currently editing (after a two-month hiatus to self-publish my wedding planning ebook), I ran into a point where the main character experiences a gut-wrenching event that causes him to gather up all of the firearms he has, loads them in the back of his Ford Explorer, and drives away to try catch the killer. Today, I sat back and examined this: would that be realistic? As a former law enforcement officer, I know this is not realistic. Cops don't wear three or four firearms when they're going out to catch a serial killer. They have one (one they are very familiar with and have shot thousands of rounds through) and lots of ammo clips.
Hence, I had to cut out all of the other firearms and narrow it down to just his main firearm.
Why? Because it wasn't realistic.
Look at your story (this is especially helpful in the editing stage) and identify areas that don't seem realistic in the context of the story. Then, imagine ways of correcting it.