As far as McDonalds USA if you search their ingredients you will see they do coat their fries and hash browns in non halal animal fat. So this would make it unlawful for us.
As for other restaurants we can eat the halal and vegetarian options as long as we are not aware the food has been cross contaminated. Most of the times they would not do that because of threat of being sued by vegans or other religious groups who do not eat meat.
Allah knows best.