I don't think there is a way to find out if an external hard drive has been plugged into some other computer that you do not have access to read the system logs.
I think the best way would be to keep track of the modified dates for the files on the HDD to see if anyone touched them.
ls -la /path/to/externaldrive > ~/hdd_change_log01
then, after someone else uses your external drive or every day (whichever suits your need), do:
ls -la /path/to/externaldrive > ~/hdd_change_log02
diff ~/hdd_change_log01 ~/_change_log02
and see if there are any changes, like modified date and file size.