Four quick‐test methods (the Foodoil sensor, RAU‐Test, Fritest® and Spot test) and two ordinary laboratory methods (the free fatty acids and GLC determination of triglycerid dimers) were compared to a standard method (column chromatographic determination of polar compounds). One hundred samples collected from fast food shops and restaurants were included in the investigation. The Foodoil sensor showed the highest correlation to the standard method (c. f.=0.94). The RAU‐Test, Fritest® and Spot test were also well correlated to the standard method. It was easier to compare the colour of the reaction mixtures to the colour scale of the RAU‐Test than to that of the Fritest®. The amount of free fatty acids were found to be an unreliable indicator of deteriorated frying fat. The triglyceride dimers could not be quantified, but assessment by visual comparison of the peak pattern in the chromatogram corresponded well with the results of the standard method.