What is main test for heart strength? echo? what about the cardiac output?

Echo: Echo is the simplest, easiest, cheapest, fastest way to determine the heart's strength ("ejection fraction"). Cardiac output, which is the product of heart rate and stroke volume (the amt of blood ejected per beat) can be calculated. Mri is more accurate but far more expensive and takes longer. Cardiac catheterization is the oldest way & still works fine for both but is invasive & more expensive.

Echocardiography: There are other modalities that can calculate "heart strength" (how well is the heart pumping blood) and cardiac output (how many liters of blood are pumped per minute) but the most accessible and non-invasive is an echo. By measuring the heart when full of blood and then when fully contracted you can estimate the effectiveness of the "squeeze" as well as the volume pumped.