How do I know if my umbilical hernia has strangulated?

See below. Incarcerated hernia means it is not reducible (unable to push it back). Strangulation happens when blood supply to herniated organs is cut off. It is a very serious problem and can lead to gangrene within hours (needs emergent surgical intervention). When a hernia is strangulated the pain increases and gets worse over time. At late stages there might be redness of the skin overlying the hernia.
Strangulated hernia. A hernia is called strangulated if the contents are squeeze so tight that the blood supply is compromised. This causes severe pain, and usually swelling, redness and it will feel warm.