How do you find an air conditioner freon leak?

The two basic ways are: 1. charge the system with refrigerant containing a tracer dye, then use an ultraviolet lamp to find the dye at the leak point. 2. use an electronic leak detector, commonly called a 'sniffer' to find the leak. Pressures in an air conditioner can be dangerous. Working on the system requires specialized knowledge. Get some help from an experienced refrigeration tech.