Thats what I see people test most, is by squirting a lot of water on the screen and let it run off. Unfortunately for me, I bought into that and got Carlack68. In the real world, it doesnt rain like a hose pipe it rains in tiny little droplets when lightly raining and thats what sticks to this kind of 'sealant' at low speeds so you have to use wipers. Then they will smear and the wipers will shudder.
What is a perfect test is treat the screen as normal. Get a bottle with a fine mist spray head and spray water on the screen. Then sit back and watch it stick.
Then go for a drive in a built up area (where we spend most of our time driving) and watch it move slightly but not enough to clear your screen, then say oh fu*k as you ram the arse of white van man cause you cant see nothing.