It is printed on a sticker or metal plate the maker calls the rating plate, together with the serial number. You want the line marked Model, or MOD, or Model No. Copy everything on it, including letters after a slash or at the end: that suffix is the revision, and it decides which part fits.
Open the door. The plate is on the edge of the tub opening, left or right side, or along the top edge of the door itself.
Top-load: under the lid, on the rim of the tub. Front-load: open the door and look around the door opening. Some brands also print it on the back panel.
Open the door. The plate is on the frame inside the door opening, or on the back of the cabinet.
Inside the fresh-food compartment: on a side wall, on the ceiling, or behind the crisper drawers.
Open the storage drawer or the oven door and look at the frame. Lift-up cooktops have it under the top.
Open the door. The plate is on the frame inside the door opening, or on the back or side of the cabinet.
A Whirlpool, KitchenAid or Maytag model often ends in a digit that is the revision: WDT750SAHZ0 is revision 0 of WDT750SAHZ, and we write it WDT750SAHZ/0. A /1 can take a different pump.
Bosch, Thermador and Gaggenau print the revision after a slash, like SHPM65Z55N/01. The number after the slash is part of the model.
Samsung and LG models carry a suffix after a slash too, usually /AA or /US. Include it.
The serial number is a different line. We never need it.