Signs and symptoms of an early miscarriage (or chemical pregnancy) would generally be spotting or vaginal bleeding similar to a menstrual period, and possibly some abdominal cramping. The bleeding might have more clots than a menstrual period; the clots would look like tiny lumps in the vaginal discharge.
If you have had a positive pregnancy test and are now having miscarriage symptoms, call your doctor for advice. Your doctor will most likely run some diagnostic tests (such as serial hCG blood tests or an early ultrasound) to determine whether or not you are having a miscarriage. Bleeding in pregnancy does not always mean a miscarriage, but your doctor should be able to figure out what is going on after analyzing the results from the tests.
Remember that bleeding or spotting accompanied by severe abdominal pain or dizziness could mean an ectopic pregnancy, so treat these symptoms as an emergency.