Cover and cook on low for 3 to 4 hours or on high for 1 to 2 hours. Check at the 3 (on low) or 1 (on high) hour mark to ensure the pasta does not over cook. To check for doneness, use a fork to pull a "test" noodle from the center of the slow cooker. Once it is tender, the pasta is done.
Cover the slow cooker and cook on high for 15 to 25 minutes, or until the spaghetti is al dente (tender to the bite, but with a slight firmness in the center). You have to taste the pasta to make sure it is properly cooked.