This paper examines current best practices for Digital Forensic (DF) tool and method validation in the context of file system interpretation for digital evidence. In order to meet the legal and scientific requirements in criminal procedures file system (FS) reverse engineering (RE) is a necessity. Currently, there is no standard procedure for reliability testing of FS RE. Ideal validation requirements exist, but they are on high-level and practical implementation is missing. In this paper we propose a formal reliability validation procedure for file system reverse engineering, documenting the forensic process, including the tools used, ensuring reliability and reproducibility of the method and the results. The procedure is based on legal and scientific criteria and tested against file system reverse engineering methods. It is applicable to all types of reverse engineering methods in digital forensics.