A recent analysis of B →π K decays concludes that present data do not clearly indicate whether (i) the standard model (or Δ I=0 new physics) is sufficient, or (ii) Δ I=1 new physics is needed. We show that these two possibilities can be distinguished by whether a sum rule relating the CP asymmetries of the four B →π K decays is valid. If case (i) is favored, the sum rule holds, and one predicts A_CP(π^0 K^0) = -0.15, while in case (ii) fits to new physics involving large values of a

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... ssed tree amplitude entail A_CP(π^0 K^0) = -0.03. The current experimental average A_CP(π^0 K^0) = -0.01 ± 0.10 must be measured a factor of at least three times more precisely in order to distinguish between the two cases.