300wsn is right about the colour of the velvet, but Manchurian are also slightly bulkier in build than a Jap, and the antlers on a Jap tend to be more straight up than a Manchurian, which lay out to the side slightly.
In Scotland where I have my lease, the Japs all originate from a deer farm that went bust during the WWI and I believe again in WWII. They are all Japs, there are no Manchurian.
On the area I have in Dorset, thay are again all Japs, I am not aware of any Manchurian blood lines. Cornbury Park does have Formosan Sika, and I have no doubt that some may have escaped into the local area. Sika are hard to keep in or out at the best of times, and I have personally seen a Sika Stag jump a 6ft deer fence without any problem.