How would someone know if the balloon deflates while it is in their stomach?

Your urine will turn green from the blue dye that is placed in the balloon.  This is temporary and harmless. As the blue dye combines with your urine you get a color of green in the toilet bowel.  You should be seeing your doctor as part of health and fitness program as well.  If this does happen you should make sure you contact your doctor.

There are a few signs and symptoms that may indicate that a gastric balloon has deflated while still in the stomach. These include:
Sudden weight loss: If you experience a sudden and unexplained weight loss, it could be a sign that the balloon has deflated.
Abdominal pain: Discomfort or pain in the abdominal area, particularly a sharp or cramping sensation, could be a sign of a deflated balloon.
Nausea and vomiting: These are common side effects of a deflated balloon, as the deflated balloon may irritate the stomach lining.
Changes in bowel movements: Constipation or diarrhea could occur due to the deflated balloon obstructing or interfering with regular bowel movements.
Visible signs of the balloon: In some cases, the deflated balloon may become visible through the abdominal wall, especially if it has migrated to a lower position in the intestine.
If you experience any of these signs or symptoms, it is important to contact your doctor immediately. They will be able to assess the situation and determine if the balloon has deflated. If the balloon has deflated, they will need to remove it to prevent any further complications.
To prevent a gastric balloon from deflating, it is important to follow your doctor's instructions carefully. This includes avoiding foods that could damage the balloon, such as sharp or hard objects, and not lifting heavy objects. It is also important to have regular checkups with your doctor to monitor the balloon's progress.

The balloon takes up space in the stomach and it is normal to feel it. with time the person gets used to it and sometimes stops feeling it. if there have been cases in which the balloon loses its volume and deflates, this case can go unnoticed by the patient, because the balloon material is designed to be disintegrated by the body.

There are patients who report having seen residues of the balloon in their stool, but the possibility of appreciating it is very low.

usually after 4 months the body takes care of disintegrating this material and the saline solution used to fill the balloon is re-absorbed in the body.

Not every surgeon uses blue dye on the balloon so you won't see any changes on the urine. The classic symptoms are weight regained, loss of full sensation or early satiety.