When you're working out you should always weigh yourself before a workout to get the most accurate results.
If you weigh yourself after a workout you won't be getting the most accurate results as you would be losing a little weight temporary due to fluid loss after working out.
But that little amount of weight will return soon so weigh yourself before the workout to get a more accurate weight reading.
Some people weigh themselves after a workout as well but only if they are trying to keep tabs on their fluid losses.