It is a good piece of information to have available to help you with feed or medicine.
Here is a simple formula to figure it out without taking them to the weighing scale.
Measure the horse around their heart girth which is 4" behind the front legs, then measure their length from the withers to the base of the tail. Then do this formula with those measurements:
Heart girth X heart girth X length divided by 330 and that should give you your weight.
Let me know if it works for you.