The model or serial number for an Air Handler can be found on the information sticker or tag, which is usually located on the front panel of the unit. The tags are sometimes placed on the inside of the front panel or can be on the original sale invoice.
The location of the Air Handler model number can depend on the type of unit. Diagrams of usual locations for the tags can be found online on the company's official website. Air Handlers are often large units, and usually contain a blower, elements for either heating or cooling the air as it passes through, filters and sound dampeners.