Visit the school's website. Click on the "Admissions" link on the page then look for a link that says "Application Status." If you still need to turn in additional information for the application, do so as soon as possible to avoid delays. You can inquire about the status by emailing the admissions director.
Meet with your undergraduate admissions counselor. Tell her you filled out an application and you're trying to find out its status. If she tells you she received your application and is reviewing it, ask how long it will be before you receive a notice of acceptance or rejection.
Call your admissions counselor. You'll need to have your application information on hand. Speak respectfully but don't be afraid to ask questions if you're unclear about anything the counselor tells you.