I think you can only do the practice exams in the test center if you are enrolled in a course, but maybe this varies by area? However, both Kaplan and TPR offer free half-length tests periodically, check their websites to see when they are doing this near you. They do this as a promotional thing for the courses, but they are proctored and they give you feedback on your performance later, so it can be worthwhile. I took 2 of these from Kaplan, I heard they purposely make these diagnostic exams more difficult to scare you into taking their courses but did not actually find the ones I took to be substantially different from other practice tests, maybe a little more difficult.