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/