You can always use the normal tagging system, but anyone can set anything there so it wouldn't guarantee something is original.
There are several hints that tell you whether someone added their own content to an image using Imgflip, such as the EXTRA IMAGES ADDED and IMAGE DESCRIPTION on the image page. In addition, the Approval Queue now displays box outlines overlaid on the image so moderators can see what exactly was added more easily.
But if someone uses purely photoshop, it's impossible to determine whether the image is original without doing an exhaustive internet search.
Caption this Meme
