One of the first signs of a miscarriage is usually loss of blood from the vagina. Pregnancy signs such as tender breasts and nausea sometimes decrease right before a miscarriage. If the miscarriage persists it is usually accompanied by abdominal pain. The intensity of the pain generally depends on the duration of the pregnancy. In the beginning the pain can resemble menstruation pain and can increase to labor pain.
On the other hand, a miscarriage can also occur without symptoms, or with a little (brown) blood loss and minimal cramping.
Besides that blood loos doesn't always mean a miscarriage is happening. An ultrasound can usually provide a definitive answer.