Try the Dual-core optimizer or the amd cpuinfo utilities, then press ctrl+alt+del and look at the performance tab, and then see if it shows both cores.
http://www.amd.com/gb-uk/Processors/TechnicalResources/0,,30_182_871_9706,00.html
http://www.amd.com/gb-uk/Processors/TechnicalResources/0,,30_182_871_9706,00.html