Other people could do a way better job of explaining it, but when you shift chainrings, the rear derailleur cage needs to pivot forward or backward to take up the chain slack. And if the derailleur itself is straight, but the hanger is bent, then the whole derailleur will move at an angle in/out rather than straight forward/backward. So without any fancy gauges, I will check my indexing up and down the cassette from above in one chainring, then shift to the other chainring and do the same. If something is bent, I'll see the chain come to the cassette cog from a slight angle, and chances are good it will index better on one chainring than another.