I've tested two methods with flan. The first suggests using a knife inserted into the center of the flan, but I do not recommend this method. I find that the knife still came out with flan on it even when my flan was over-cooked. It is not a reliable method. My favorite method is using a toothpick. Insert into the center of your flan. It should come out moist, without many pieces of flan sticking to it. Keep in mind your flan will still be decently jiggly when it comes out of the oven, but will firm right up as it cools down.