I always run short and have to add. I used to be a short story writer and tend to write too spare. With my last novel, I had to add 15K just to bring it up to minimum word count. That was hard! I had to review the entire book to figure out what to add--and more importantly, it couldn't be filler/padding. It had to be important material.
This is what I looked for:
Sentences/paragraphs that are unclear or confusing or that more could be done with. I think this was the majority of my additions. Sometimes that paragraph led to an entire page to flesh it out better.
Anything that didn't quite work. One chapter escaped notice under multiple revisions, and while looking for expansion points, I discovered the chapter didn't have a whole lot going on in it. But the piece it did have was important to the story. So I expanded it--into three chapters that were much better than the original.
Dialogue is another good place to look. I didn't look for additions hear under my last book, but I will under my WIP.
Anything where you have a weakness, work on it. Because of my short story background, I tend to leave out detals, so I always have to add to them. It's likely to be description of some form--though I'm never going to be too long. People still tell me it's spare, and I'm thinking "This is pretty long!" I'll be doing more of this on my WIP also.
I think the key is to treat it as another revision to make the story better. Focus on areas where you know you're weak and running short and work through those first.