For TVs:

You can usually find the model number on the back of your TV, located on a sticker or on the box it comes 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 iPads and tablets:

Generally, you can find this by accessing the Settings or Options app, then click About, or similar. The model of the device will then be displayed there.

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.