For TVs:
You can usually find your model number on the back of your TV located on a sticker, or on the box that it came in.
If your TV is mounted and you can't see the back, you may also be able to find the model number in your TV's menu options or the device's user manual.
For phones and tablets:
Generally, you can find this by accessing the Settings or Options app, then click About phone/tablet, or similar. The model of the device will then be displayed.
For anything else:
You can usually find your device's model number on the box it came in, or on a sticker located on the back or bottom of the device itself.