Is there any way to tell how much weight a pregnant woman has gained in her belly before actually giving birth with out weighing the belly?

Do you mean weigh the baby before birth?

You can obviously weigh the mom, and see how much weight she has gained, but the weight isn’t all in her belly. Her breasts are bigger, her blood volume has increased, she may have increased her fat storage all over her body, and the amniotic fluid weighs something, etc.

My cousin gained 79 pounds and then delivered a 8 pound 9 ounce baby boy. I’m sure doctors have a way, but I don’t know how they do it.

Located this website to get an approximate .
Depends on type of body etc
https://www.whattoexpect.com/pregnancy/weight-gain/

Usually the doctor will measure your growing abdomen and give an estimate of how big the baby will be. A pregnant woman should have prenatal care.