How Can I Tell Whether an Organic Traffic Drop Is Seasonal or Search-Related?

My site’s organic visits have dropped over the last few weeks, but I’m not sure whether to treat it as a search visibility problem or a normal seasonal dip. The pages affected cover topics that tend to get more attention at certain times of year.

When you’re investigating a change like this, what comparisons are most useful? I’m especially interested in how to separate a broader demand shift from a change affecting particular pages or queries.