Close the curtains. Turn on the TV.
The weatherman (or –woman) will show you maps of Europe with lines and the arrows at which he/she will wave significantly. Totally irrelevant to whether you should go out with an overcoat or a brolly, but you just have to wade through it to get what you want.
Eventually your patience will be rewarded with a grit full of suns or clouds, with or without dotted stripes, and numbers.
Fortunately the weatherman (or –woman) will explain all the suns and the clouds and the numbers, so you know what to expect the next few days.
In case the weatherman (or –woman) is wrong: Off with his head, or to be more modern: Sue the company.
In the country:
Open the curtains and look up at the sky.
Assess the direction of the wind. To help you, plant a flag in your garden or on top of your house.
Tap on the barometer to see if the pressure is rising or falling. Rising: take a sunhat with you. Falling: umbrella or raincoat.
In case you are wrong: tough luck!