A quick way to measure if your rear derailleur is bent is to remove the rear derailleur and the front wheel. Thread the axle of the front wheel into the derailleur hanger if it's the same thread. Otherwise, you can clamp it on with the quick release or bolt it on.

Then just eyeball it or measure the distance from the rim of the front wheel to the rear wheel. You can also use the front wheel to lever the derailleur hanger straight again.