Regarding this: most stolen work is stolen from quite popular models. If you want to be as sure as you can be (and you would clearly like that), just do a quick search on google images for the object followed by “STL” and if it is indeed a popular model you most likely find different websites with the model of which one or more are the original designer. Just look at the dates and you can be pretty (not 100%) confident you are talking to the right person. Most thieves even copy the exact photos, title and description. It is a bit cumbersome but sadly there is no easy guarantee