You'll sometimes see someone tag a picture post, if they tag it at all, if it's something the blogger actually created themselves with #oc for "original content" or they'll just tell you as much in the caption. Not always, but sometimes.

I'm not on tumblr for the porn (I read it for the articles, I swear), but a lot of the mechanisms and meta-memes of how people use the platform are pretty consistent across subgroups.