Ideally pasta should be cooked until it is al dente, which means that it is toothy. The reason for this is that the pasta will continue to cook when it is removed from the boiling water and tossed with it's sauce. Cooking it to this point will ensure that it is not mushy when served. But to know if the pasta is fully cooked you can check a piece of pasta by cutting in in half. The center of the pasta should be uniform in color and there should not be a white ring in the middle. You can also taste it. It should be tender but not soft.