Reliable test methods for checking cleanliness are needed to evaluate and validate the cleaning process of fermentation tanks. The size of a fermentation tank can be several hundred cubic meters limiting the use of many traditional sampling methods since the reachable areas are scarce compared to the entire tank surface. Pilot scale tanks were used to test the applicability of various methods for evaluation of the cleanliness of fermentation tanks. The aims of this study were to find appropriate methods to detect cleanliness in pilot scale tanks, to find out which parts of the tank were difficult to clean and to optimise the cleaning procedure. This study was performed to improve the investigation of cleanability of process scale tanks and to choose suitable methods and sampling areas for use in process scale evaluations. Furthermore interpretation of data from various sampling methods is needed in cleanability assessment. The methods found to be suitable for validation of the cleanliness were visual observation of a fluorescent indicator using UV-light, a contact agar method and culturing based on swabbing and swiping with non-woven cloths. The validation of the cleaning procedure and the design of a proper cleaning system can be supported and improved using computational fluid dynamics, which through a simulation reveals areas not easily covered by the cleaning fluid. Microbial results from pilot scale studies are needed to interpret the simulation results properly. Using various methods simultaneously improves the interpretation of cleanability of tanks.