Balancing weights and trim tabs. If your mechanics have good software, hardware and experience, they can match two, three or more unrelated blades. Not a problem.
You forgot to mention sweeping, & some blades have more than one fore/aft position for the tip weights (thereby effecting the cordwise balance). Many have inboard and outboard trim tabs too. Yes, as stated above, the 'new fangled' balancing equipment is "supposed" to make it so a monkey can track & balance a ship in just a couple of hours. That is not always the case & a thorough understanding of the blade/blades you are dealing with is not only essential, but may save you days, or even weeks of chasing your tail.
There are many things also to consider than just getting the ship to fly smooth on the ground, in a hover, high speed flight. How about what happens when you let go of the collective? what does it want to do? rise? fall??,,,,,,,,,,,do these new computerized balancers tell the mechanic that?. Just because you own the latest & greatest balancer, does not guarantee adequate results.
Like 500 said above?, how many have actually weighed a blade B4 trying to match it with the others? not only the overall weight of the blade, but the tip, the root & the chordwise? Do many even know HOW to check the chordwise balance? What will it effect if not the same as the others? Aerodynamic and mechanical twisting moments?
I don't claim to be an expert, but i have had enuf balancing problems to know that there is more to it than just a pitch link adjustment, trim tab, balance weight, sweep, to make it fly acceptable in all regimes. And YES, you will come across a stubborn blade that no matter what, will refuse to fly nicely with the others, what do ya do then?
Not sure what the OP had in mind with his question, but? i am assuming that he wanted to know "how ya know", B4 ya waste to much time on the project.