As steel cord conveyor belts are vulcanized during the production process, rubber penetrates into steel cords. It is a positive and a desired phenomenon allowing the belt to reach its required properties and ensuring its reliable and long operation on the belt conveyor. Advantageously, the rubber compound penetrating into the cord improves the strength of the cord-rubber joint by increasing the active contact surface of the two materials. This fact is of great significance in the view of the steel cord conveyor belt splicing technology, which relies on an adhesive connection between the cord and the rubber. Moreover, filling the inside of the cords with rubber protects against corrosion and has a good effect on their durability. The article presents a laboratory method for testing and evaluating the penetration of the steel cord with rubber. The method has been developed on the basis of numerous laboratory tests, which are also here described. The presented method is reliable and can be used for a broad range of cord diameters and designs. The proposal also includes a quantitative measure of the rubber penetration of cord and the criteria for its identification.