How to know when milk has curdled enough to use for baking?

I decided to bake today and for my recipe I need to curdle milk using 1 cup milk and 1 tsp. distilled vinegar. I've let the milk and vinegar set for 15 minutes but I'm not sure it's curdled. How do you know if your mixture has curdled? Mine isn't very thick, is it supposed to be thick or thin?