doing a stage is a great way to get a feel for what restaurant kitchens are like and most high-end restaurants are fairly used to people doing stages.
call the restaurant, ask for the chef, and tell them why you're calling. if the chef's not available, ask if you can leave a message or fax/email a resume.
make sure you tell them that you have no restaurant experience so at least they'll know that going in.
if you get the stage, wear appropriate clothes and shoes, don't count on them supplying you with a chef coat, bring your own knives, and be prepared to do what they tell you to do AND ONLY what they tell you to do. don't talk too much. don't ask a lot of questions. lay back, watch, observe, and be ready when someone throws a task at you (they'll only give you tasks that a monkey could do).