While you can certainly weigh your luggage on your regular scale to get a reading, an easier way to get an accurate weight is to use a luggage scale. Just strap the scale to the handle of the bag and lift. Typical luggage scales, like the one here, go up to 50 kg or about 110 pounds. Overweight fees vary by airlines but can start at as low as 50 pounds.
Disclosure: I received the above mentioned product for the purpose of this review. No other compensation was received. All opinions stated are my own and may differ from yours. See my disclosure policy for more information.