Some custom knives are made with the blade slightly offset from the tang. Randall are famous for this.
Lay a ruler on the blade and look to see if there is any light showing underneath, from the choil to the tip. That will tell you if the blade is straight. The set the ruler on top of the knife, on the ruler's edge, lined up with the tip of the blade and the middle of the handle. This should tell you all you want to know.
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