Dress ready to interview on the spot, take a resume and cover letter with you. I would always go in and had the receptionist my resume, let them know I had heard they may be looking for someone, an then ask if there was anyone available for me to talk to and get more information from. Don't be too put off if they just take your resume and tell you no one is available at the moment. Some clinics don't like to talk with applicants until they've had a chance to look at all the resumes and applications.