An alignment tool is the best way to go. That said the accuracy of bending a steel hanger back is "somewhat" dependent on the number of speeds you are running.
Some years ago I used a precision square in the dropouts and an 18" crescent wrench to straighten a bent hanger. I think the use of a precision square over a Harbor Freight square is the key, at least in one's own mind.
But I'm running an 8 speed cassette and the Centeron probably picks up enough of the visual out-of-tolerance.
If I were doing it today, I'd get an alignment tool. I have a lot more tools now than I used to have, and many only get occasional use. I somehow survived my ham fisted era.
John