That's how found. Lead is a heavy metal that was used as an additive in paints & gasoline in the past. It is also found in many industrial settings. Trace amounts are usually not a problem but it can be dangerous. It interferes with the formation of blood, which is one basis for blood tests. It can also attack nerve cells and cause brain injury. The simplest way to screen for exposure is a blood test.
Lead is toxic. Lead is in our environment and many children are exposed to it in many different ways. Lead can hurt your brain, it can cause anemia and many other effects. So it is good to screen children early in life. If it is high it can be remove from the body by special medicines.
Yes. Toxins in the blood stream can be largely tested for.
Yes. It can show directly or indirectly on more than one type of test. Lead can interfere with blood formation and trigger anemia. It can also be measured specifically in blood or serum. If present for an extended period in growing kids it can show up as lead lines in the bones.