External Hard Drive last used / files copied on another computer?
In short, No.
There's no way to be absolutely certain.

Files could have been copied from it, the entire drive could have been cloned without leaving a trace.  

You can have a look at the Last Opened data, but it's not conclusive proof of a negative.
You'd have to check a folder at a time [Search ought to be able to do it, but it never seems to work properly other than on "This Mac" which would show you too much other data]  

Open a folder on the drive, set it to View > As List.
Right click the columns bar at the top &amp; add Date Last Opened.

Click the new column to sort on it.
Check folder by folder...



Of course, anything you have opened since will confuse the results.