The beginning (first 3 segments) of the mac address shows if a interface is virtual:
"00:05:69"; //vmware1
"00:0C:29"; //vmware2
"00:50:56"; //vmware3
"00:1C:42"; //parallels1
"00:03:FF"; //microsoft virtual pc
"00:0F:4B"; //virtual iron 4
"00:16:3E"; //red hat xen , oracle vm , xen source, novell xen
"08:00:27"; //virtualbox
EDIT
To be more clear, if a interface has a MAC address that starts with any of the above given strings, then it's virtual.
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