There are two simple test for braking power.

The rear brake should be capable or locking the rear wheel on dry level pavement. If it can do that there's nothing more it can do, since it's braking power exceeds the tire's traction. BTW- this is a mandatory requirement in most states, and also under years old CPSC requirements, so any brake that cannot is inadequate, and things like replacing shoes, cleaning the rims, replacing cables might help. In some cases cheaply made calipers need to be replaced.

The front brake should be able to apply enough stopping power to lift the rear wheel off the ground. Here again, it's the most it can do, because any more stopping power will flip the bike. This test obviously has a degree of hazard, and should be approached with caution, increasing braking effort by degrees over multiple tests, until you're either happy with the brakes, or feel you've reached the tipping point.

If both brakes are capable of achieving maximum braking power, the other issues are modulation and predictability, which is what separates better systems from worse, and rider technique. With practice a rider develops technique for proportioning his braking effort properly between both brakes for effective, controlled braking.