If you really can't differentiate, you can always read nutrition facts on the food(mainly the ingredient of the dish) that you wanna know its content. As for french fries, I think as long as you deep fry them yourself, the trans fat is literally negligible. The problem with deep-frying food outside is that they keep reusing the oil which already has trans fat in it and it will keep increasing because heat energy promotes reaction(hydrogenation) in the molecules to stabilise themselves. So if you really crave for food like french fries, just deep fry it yourself with new cooking oil every time. You may reuse the oil 1-2 times but more than that is strictly not recommended.

One way to roughly guess whether a dish has trans fat or not is to see the cooking method itself involves high temperature or not and how many times the cooking oil is used. So from these 2 principles, you can tell stir-frying food and deep-frying food have trans fats but not that much(or negligible) unless the cooking oil used is reused many times already.

Whenever the cooking oil is heated, be it new cooking oil or reused oil, trans fat will be produced gradually from a reaction called hydrogenation. And, high temperature will increase the rate of this reaction as the molecules of the cooking oil have more energy to attach to the hydrogen atoms in the oil to stabilise themselves. If you use new cooking oil or oil that has been re-used once or twice, the trans fat produced in it is not that much to the point it harms your body because even the 'boosted' rate is still slow, so it's considered as negligible. Whatever inside the cooking oil will stick to the dish you're preparing, so your dish will have trans fat too, it's the amount of trans fat that matters.

edit: The type of cooking oils you use also matters. One of the healthiest cooking oil is peanut oil. You can literally use peanut oil for every cooking method as it has a very high smoking point and high in unsaturated fats. If you want the healthiest cooking oil, it'd be sunflower oil with high oleic, but I believe it's also one of the most expensive cooking oil.

From this table, you should pick the cooking oil that has high monounsaturated fats or high saturated fats for frying method(deep or stir), of course, the preferred cooking oil would be the one that has high monounsaturated fats.