Most of them, luckily, are vegan. But in some cases, they can contain beef fat or are fried using the same oil as chicken wings or other animal products! When eating out at any non-vegan place, you need to accept that there is very likely some food contamination going on and traces of meat or dairy products can be found on or in your food. If you buy frozen fries at the store, you can check the label for non-vegan ingredients; most of them will, at least accidentally, be vegan.