Unfortunately the answer is no. You also can’t make sure that everything that you buy or eat is made 100% ethically. But it’s what you do most of the time that matters.
I went to Olive Garden today with my family after going to the Mac store to repair my laptop. I got soup, and veggie penne with garlic mariana sauce. I also had a little bit of the salad that everyone else got. The one without graded cheese, which I don’t understand why anyone would put cheese on a salad. The soup didn’t have any meat, dairy or eggs on it. The veggie penne had a little bit of cheese on it. Not very much cheese but enough that you can see that it exists. So I ate a little bit of cheese.
Nobody’s perfect but if you make a conscious effort to reduce suffering then that is what counts. And that is, in my opinion, what being vegan really means. It doesn’t mean that you will never accidentally eat a little bit of dairy or egg every now and again. (Meat is something that I won’t eat any.) It means that you know where you food comes from, how it is made, and you make the effort to reduce the amount of suffering that you cause in the world.
I used to beat myself up for eating a little bit. But eating just a little bit by accident is better than eating bacon and eggs for breakfast, pizza with dairy cheese for lunch, and steak for dinner on purpose.