Best tool is to know that you tend to do that, and think about doing it correctly.

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Swimming Workout of the Day:

Favourite Swim Sets:

2020 National Masters Champion - M50-54 - 50m Butterfly

Try swimming some lengths breathing every 5 or 7 strokes so you can worry about the deficient arm the whole way through the stroke. At other times breathe with whatever pattern you want but ignore your left arm and only think about what the right one is doing.

To dumples' point, a snorkel can help you focus on the offending arm by taking the breathing mechanics out of the picture. Apparently the Texas Longhorns warm up every practice with the snorkel. If they're doing it, I'm guessing there must be something to it.

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Swimming Workout of the Day:

Favourite Swim Sets:

2020 National Masters Champion - M50-54 - 50m Butterfly

http://www.theraceclub.com/aqua-notes/is-the-high-elbow-pull-best-for-the-freestyle-swim-stroke/