Carrier puts the model number inside the front door on each furnace. Carrier also puts the model number on the paperwork that is generated with each purchase; one example is the owner's manual.
A decal or rating plate should be inside each gas or oil furnace made by Carrier. To access the decal or rating plate, remove the front door from the furnace and view the model number. Carrier says that it tries to make model numbers easy to find by putting them in easily accessible locations. A local Carrier dealer can also help you find or distinguish what model a Carrier furnace is.