TheVMinator wrote:
But what if I need to know after it happened whether xcopy was used?
You might be able to tell later by study the historic performance charts for your storage adapters. If you see large number of reads and write (equal to the size of the VMDK) at the time of the Storage vMotion then VAAI XCOPY was not used... But likely not a practical way. 
TheVMinator wrote:
Or how can I tell based on current configuration settings before it happens whether xcopy will be used?
If you see the "Clone" primitive as supported it will be used, most exact view is from the command line: http://rickardnobel.se/verify-vaai-from-gui-and-command-line