Validating an end-entity X.509 digital certificate prior to authorizing it for using a resource into the computational Grid has become a widely studied topic due to its importance for security. A more comprehensive validation process involves not only a real-time check on the credential's status, but also an evaluation of the trust level applicable to its Certification Authority. Nowadays Policy Management Authorities (PMAs) gather Grid CAs fulfilling a minimum set of requirements defined in an Authentication Profile thus guaranteeing a trusted interoperability environment for Grid projects. Currently this is a manual process that only results in a binary decision (the CA is able to become part of the PMA or not), however in practice, different CAs offer different security levels. In this paper we present ways to apply the Reference Evaluation Methodology (REM) to automatically obtain the security level of a CA. The described process is based on the building of a formalized policy template for Grid Certificate Policies. This methodology has been used to evaluate the security level offered by a set of EUGridPMA's CAs; the obtained results are then conveyed to relying parties using an infrastructure composed of CertiVeR's Validation Service and the Open GRid Ocsp (OGRO) middleware for the Globus Toolkit 4, thus providing enough information for a comprehensive certificate validation decision.