A red dot in the lower right corner automatically identifies all paintings that have sold on DPW. Any listed as "sold" in your Art Tracking grid also will have the red dot. You can also create a "Sold" category in your gallery, then just add a "sold" tag to your paintings, and they will all show up there.
Sales are up and down. I am currently taking a few months off to build up some show inventory, but sales are best when I post something new at least weekly and participate in the weekly challenges and monthly contests. More exposure=more sales.