It REALLY helps if you have very bright, small light that uses blue LED's (such as the 5 LED light by INOVA as it has a fair amount of "UV") ...clean the viewing slot with a rag, pull off inspection port cover mentioned above, place the bright light right ON the overflow bottle...and view level thru the vertical slit ...with OEM antifreeze AND the blue light, one should see day-glo green fluid about half-way between the two embossed lines on the fender splash guard by the slit (with the engine COLD)...and the air above it will appear purple. This method works well even in bright sunlight with the proper light source.