How can I tell how many Cores SQL Server is actually using?
Express edition is limited by 1 processor, not 1 core:


  SQL Server supports the specified number of processor sockets
  multiplied by the number of logical CPUs in each socket. For example,
  the following is considered a single processor for purposes of this
  table: A single-core, hyper-threaded processor with 2 logical CPUs per
  socket. A dual-core processor with 2 logical CPUs. A quad-core
  processor with 4 logical CPUs. SQL Server is licensed per processor
  socket, and not per logical CPU basis.


http://msdn.microsoft.com/en-us/library/ms143760(v=sql.105).aspx

And yes, sys.dm_os_schedulers can be used to ensure, that all 4 cores are mapped with schedulers.