If Dell (or their sub-contractor) have done it right then the center pin or PE should be connected to negative on the output, this means that the chassis of the notebook is then grounded. Notebook supplies are however strange beasts and at least half of them from any manufacturer will have the output floating, with ground only used for the filters on the mains side and maybe the screens.
As a test measure using a 10M impedance voltmeter between negative output and mains earth with the supply running. If you see half mains voltage then your output is probably floating. Note that this voltage is normally quite safe, it's just leakage current from the capacitors.