Confirm Unmounting/Ejection of Drive During Shutdown in Logs for Yosemite 10.10.4
Not sure if this will be of much help however...

With Console open and system.log selected I ejected a USB Drive and removed it from the USB Port.  By all outward appearance the drive appeared to be properly and successfully unmounted, ejected and then removed.

These are the only entries that appeared in the system.log file upon doing so:

Aug 12 10:43:21 Computer.local com.apple.SecurityServer[17]: Succeeded authorizing right 'system.volume.removable.unmount' by client '/System/Library/CoreServices/Finder.app' [51235] for authorization created by '/System/Library/CoreServices/Finder.app' [51235] (100013,0)
Aug 12 10:43:21 Computer.local com.apple.SecurityServer[17]: Succeeded authorizing right 'system.volume.removable.unmount' by client '/usr/sbin/diskarbitrationd' [18] for authorization created by '/System/Library/CoreServices/Finder.app' [51235] (100002,0)
Aug 12 10:43:23 Computer.local com.apple.SecurityServer[17]: Succeeded authorizing right 'system.volume.removable.unmount' by client '/System/Library/CoreServices/Finder.app' [51235] for authorization created by '/System/Library/CoreServices/Finder.app' [51235] (100013,0)
Aug 12 10:43:23 Computer.local com.apple.SecurityServer[17]: Succeeded authorizing right 'system.volume.removable.unmount' by client '/usr/sbin/diskarbitrationd' [18] for authorization created by '/System/Library/CoreServices/Finder.app' [51235] (100002,0)


I'd image if it wasn't properly unmounted, ejected and removed you might get a warning when you plug it back in.  Or there may be an error message in the system.log file however I do not when to improperly remove a drive to test, you'll have to do that. :)

One thing I do every once in a while, because I'm constantly moving USB Drives around, is to use Disk Utility to Verify Disk and Verify Permissions on my external drives just to keep them healthy.