What are signs of high cardiac output?

Varies: Assuming a true high cardiac output syndrome, patients may feel flushed with a fast heart rate. Most are transient situations, but some illnesses and conditions may cause a sustained high output state, e.g., hyperthyroidism, anemia, arterio-venous malformations (congenital or acquired low resistance connections between arteries and veins), or disease like paget's. Requires confirmation by doctor.

None: You can't generally feel it. In extreme cases, it can lead to congestive heart failure. If symptoms are present, they are more likely to be due to the underlying condition which is precipitating the high cardiac output state (like sepsis, fever, infection, trauma, thyroid excess, etc).