Quite a few of the source systems also supply ES membership numbers with the results during the updates each night and these can be viewed either in the player's history page or by selecting the check box 'ES numbers' at the top of the rankings pages. Note that only the appropriate admins can see a player's ES membership number.
The system checks the membership number against the ES database so can tell if the number is valid, invalid or expired. It only does this when needed (when the pages are viewed) and only does it once per week maximum to reduce the number of accesses to the ES database.
Very useful for checking if your league players are members of ES - which they should be.