Abstract: A product, which passages are to be checked is brought to the measuring position, at least one measuring probe containing at minimum one emitting element and at minimum one receiving element is moved toward or inserted into openings of the product passages or an assembly of measuring probes consisting of at minimum one measuring probe containing at minimum one emitting element and at minimum one measuring probe containing at minimum one receiving element is moved toward or inserted into the opening of the product passages, following the part of the assembly consisting of at minimum one measuring probe containing at minimum one emitting element emitting in range of visible spectrum and/or infrared spectrum in the region of wavelength from 400 nm to 1500 nm is turned on and on the measuring probe, containing the receiving element, that forms a pair with the just working emitting element a response—a signal, size of which depends on amount of passed light, is measured, a signal amount is analysed and compared wi