A wellhead assembly with an external test port for testing a primary seal at an interface of a wellhead and a base. The assembly includes unique architecture with a secondary seal provided adjacent the primary. In this manner, testing of the primary seal from an external location may proceed without concern over a false indication of primary seal failure. That is, for circumstances where a more interior side of the seal fails under pressure testing even though an outer face of the seal might remain in sealing engagement, the secondary seal would ensure that the outer sealing engagement is detected. As a result, a practical manner of testing the wellhead seal from an external location without the need of plugging the wellbore is provided.