Installers use infrared lamps and thermometers for a measured comparison, but you can get a rough sense of it yourself once the film has cured. Lower a window partway in direct sun so part of the glass is filmed and part is not, then hold your hand behind each in turn and feel the difference in radiant heat. It is not a laboratory test, but it does show what the film is doing. For published figures, look at the performance data for the exact film and shade.
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