I would try to get a sense of how productive (from a publishing perspective) the lab is. This isn't necessarily something you even need to ask - just search on PubMed and see how many papers with the PI as an author have been published in the last year. This isn't quite so applicable if you're going to be starting a brand new project or different line of research within the lab, but you will be more likely to get a publication with a PI that publishes frequently than a PI who publishes once a year or less frequently.
Another thing that you can try and get a sense of is how far along the project you'll be working on will be. As above, if it's something that is just starting, it will likely be some time before anything tangible comes from it. If you're jumping in toward the middle or end of a project, though, the possibility of something being published is more likely.