If the wheel sits off center, and mirrors when flipped it's the wheel. If it doesn't move when flipped, it's the fork. If it moves, but doesn't mirror, it's both.
However, there are other variables, like possibly a bent axle, or worn area where it sits in the fork. So paint a reference dot on the locknut, and repeat the test with the dot on top to ensure the axle is in the same place (except for flipping). Then with variables controles, the 3 conditions I listed hold true.
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