How to tell if I have a ruptured spleen?

Usually. Ruptured spleen causes intra abdominal hemorrhage and shock within a few hours of injury. In some cases, a small tear can cause limited bleeding, which can even stop. The only way to know for sure whether you have a splenic injury or not, or its extent, is to do an imaging study, such as ultrasound or ct scan.
You cannot. The patient would have serious abdominal pain, shock, even death. Usually after major athletic or vehicular trauma. Occasionally with other disease with enlarged spleen a rupture can occur.