I don't think that there is any way to get a list of tokens which have been generated/issued.You can however, change the security key which will invalidate tokens previously issued. You can also control how long tokens can be issued for as well.Do you have a specific concern/problem?
If you generate the tokens then you can manage this information yourself.I do not believe that ArcGIS Server is going to do this for you.
you can post in ESRI ArcGIS Ideas http://ideas.arcgis.com/
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