To take a practice test, you can click on the link on your Dashboard that says "Take a practice test" -- it's in the Quick Practice section, right above the green Practice Verbal buttons.
Our full length practice test questions are drawn from our general pool of 1100+ questions, so you can take about 3 full-length tests before you start to see repeats. We usually recommend only taking 1-2 and saving the other questions for more targeted practice. However, you're always free to reset your questions and keep taking practice tests as many times as you like!
You can learn more about our tests in this post on our GRE blog.
If you finish all of the Magoosh questions and are looking for more practice tests, we also recommend Powerprep from ETS :)