Mongodb: Does the stats command show me the capped size setting?
If you connect to the proper DB and execute a db..stats() in the Mongo shell you will see "capped" : true third from the bottom of the output just after index sizes. 

To figure out a collections cap size, you could use the system namespace.

e.g. db.system.namespaces.find({name : "."}, {"options.size":1})