“Preview what this would pick” checks each source's latest posts against your filters and routing, and shows the result. It takes about 15–30 seconds and posts nothing.
What you see
- Every post that would go out today, with the template it would be rebuilt in.
- Every skipped post with the reason: already posted, wrong content type, looks like an ad, duplicate story, over the daily budget, or a routing gap.
- A “Test-render” link on any pick, which turns it into a real post via Create so you can judge the output.
It's the best diagnostic when an automation seems quiet. If the preview shows everything skipped as “wrong content type”, you've probably fed a photo-only profile into a video template. News automations have no preview; their story cards already are one.