How can you tell?

If it’s the hair, there was specifically a stage of the CGI which used Henry’s head but without any hair. Possibly the second picture in my previous post is actually of an intermediate stage where they put Peter Cushing’s hair onto Henry’s head.

EDIT: This is in fact the case. Further scrutiny of the video reveals some odd artifacting of Tarkin’s hair in that particular shot. Meanwhile, Henry’s performance, both on set and in mo-cap, had him wearing a graying dark-brown wig.