You can see when a segment was last refreshed by going to Contacts > Segments and looking at the 'Last updated' column.
To update a segment with the latest data, click on the Refresh icon. All segments require manual refreshing using this button; no segments listed here update automatically at all.
When scheduling a campaign send that's sending to a selected segment or segments, a checkbox option will be available that enables you to refresh segments before the scheduled send time. Ticking this will ensure segments are up to date at the point of send.
However, please be aware that using this may delay a scheduled send by a few minutes, should the segments take a while to complete refreshing.