Weigh-ins before and after gym?

It's not uncommon in professional sport as a quick way to check hydration levels....

"It is a good idea to monitor all players body weight before and after training or playing. A 1 kilogram reduction in body weight is roughly equivalent to a lose of 1.5 litres of fluid. It's not unusual for players to lose as much as 3 to 4 kilograms during the course of a game and a loss of 1 to 2 kilograms from training is common. Use these changes in body weight as a guide to how much fluid a player should consume following training and games. This is particularly important when training and playing on consecutive days - players can quickly become chronically dehydrated when they are required to train and compete frequently and this will have a negative impact on their ability to perform and recover."
irishrugby.ie/playingthegame/fitness/nutrition/nutrition.php

I've also done it before after a long hot cycle ride.

Huge variations in sweat rates - a friend of mine drinks a third of what I need on a fast ride!
bicycling.com/training-nutrition/how-and-when-hydrate