With a hernia, what are the first signs indicating the bowels blood supply has been cut off? And how do you prevent it from occurring?

Get it fixed. This kind of problem is very serious and dangerous. If you have a piece of bowel in a hernia, it needs to be fixed asap. Don't wait until the blood supply is cut off. Pain, aching, naseua and vomiting are some indications of blood supply being cut off. This is very dangerous.
Pain and nausea. When blood supply is compromised, it is considered an incarcerated hernia. Initial symptoms are abdominal pain and nausea. The hernia is typically "stuck" where it cannot be pushed back into the abdomen. Surgery is imperative.
"Incarceration" A hernia is said to be "incarcerated" when the contents of the hernia cannot be reduced back into the abdominal cavity. If the incarcerated structure is intestine, the blood supply can be cut off; this is called a strangulated hernia. If you have a tender hernia that cannot be reduced, especially with vomiting and bloating, this is a surgical emergency.
Bowel Strangulation. One of the first signs of impending bowel strangulation at a hernia is increased swelling and pain at the hernia site. This usually progresses to severe, diffuse abdominal pain, often with persistent nausea and vomiting.