Clean tests of a small admixture of right-handed quark currents directly coupled to the standard W are still lacking. We show that such non-standard couplings can be significantly constrained measuring the value of the scalar Kpi form factor at the Callan Treiman point to a few percent. A realistic prospect of such a measurement in Kmu3L decay based on an accurate dispersive representation of the scalar form factor is presented. The inadequacy of the currently used linear parametrization is explained and illustrated using recent KTeV data. We briefly comment on the charged kaon mode.