Abstract: The subject matter of the present invention relates a method for detection of polyethylene glycols (PEG) in an aqueous solution or in a sample of a biological fluid in a qualitative and/or semi-quantitative and/or quantitative manner, the method comprising the addition of a reagent containing iodine, alkali metal iodide and an acid; and a method for assessing the permeability of the intestine wherein two PEGs of different molecular weight, low molecular weight PEG (l.m.w. PEG) and high molecular weight PEG (h.m.w. PEG) are used as markers, wherein the l.m.w. PEG is able to cross the intestinal mucosa under physiological conditions and the h.m.w. PEG is essentially not able to cross the intestinal mucosa under physiological conditions, but is able to cross the intestinal mucosa under non-physiological, irregular conditions; and wherein h.m.w. and l.m.w. PEG concentrations are measured in a sample of urine from an individual after oral administration of h.m.w. PEG and l.m.w.