I'm not aware of any way to check the parent post yourself without actually posting it. You could check where it would be posted though, if you save the image and then hit the 'post' option it will take you to a preview version of the post, and at the very bottom it will show what board it will be posted to. Thats there so you can change the board, but it will also let you know if that means its being posting to the adoptables board or the editable oekaki one. You can also exit out of that page and the image will not be posted, and will still be in your saved oekaki images, its only posted if you hit the submit button on the preview page.
Also, if there are some you don't want to keep either way, you can delete posts you made as long as no one else has posted after you, so you can color it over or something to make it clear you aren't submitting for any kind of contest right now, post it, and then it will have a link back to the original version. Then if you don't want to keep it you can just delete it right away. I also expect for the most part if whatever contest it was for is already over, it shouldn't be problem to post it just to have and have a disclaimer in the post that its not an entry and you are just posting it to save/have for posterity or something. I'm not really familiar with adoptable contests enough to know if they usually allow WIP entries or not but they could also be posted as that if the contest is still open.
Not an immediate fix but you can also make a suggestions over on the
suggestions board for a function like this to be added in the future, have the save oekaki info include a link back to the original version if its based on another drawing. There is already an 'info' section for saved oekaki drawings and posted drawings already include links back to the parent drawing so I imagine that wouldn't be too complicated to add in on the saved drawings as well.