The influence of RF electric fields on retarding field analyzer (RFA) measurements of sheath potential, V-sh is investigated. One-dimensional particle-in-cell simulations show that the RFA is able to measure reliably the rectified dc sheath potential only for ion plasma frequencies omega(pi) similar to the rf wave frequency omega(rf), while for real SOL conditions (omega(pi) > omega(rf)), when the RFA is magnetically connected to an RF antenna, it is strongly underestimated. An alternative method to investigate RF sheaths effects is proposed that uses broadening of the ion distribution function as evidence of the rf electric fields in the sheath. RFA measurements in Tore Supra indicate that the average effects of rf potentials do indeed propagate from the antenna 12 m along magnetic field lines.