The model is usually found inside the piano with the lid open, near the front of a grand piano or top of an upright piano. It's often a short series of letters and/or numbers either painted or embossed on the cast-iron harp.
Don't see it? That's okay. Some pianos don't show a visible model number anywhere. Just make sure to give us the brand, size (height or length) and serial number so we can give you an accurate value.