Analysis of weather conditions
Understanding the weather is important for a successful fishing session. There is no shortage of sites offering this kind of data, although not all of them are very accurate. Anticipating a gust of wind in summer, or a sunny day in winter, is very interesting, since fish activity depends on it. You can also analyze your own previous outings, outlining the weather conditions, the season and the number of fish caught.
As for the most interesting sites to follow, here are a few of my favorites: Windy, Windfinder, WindGuru and La Météo Agricole. By combining information from these different sites, it's very easy to guess the weather trend and know what to expect at the water's edge.

Go back
If you've never had the chance to visit a lake whose water level has fallen drastically in the last 10 years, you'll find it impossible to guess what the bottom looks like. Fortunately, Youtube and Google Earth Pro are two tools that allow us to go back in time. When dams are emptied or during periods of drought, there are often video reports or drone shots of the state of the lake. If you can find such videos, you'll be able to see what's going on underwater.
In the same way, Google Earth Pro offers satellite images from several different years, which can enable you to find a year when the water level was low, and thus offer you important information on plateau areas, breaks or the position of obstacles.

Internet documentation
Thanks to the advent of the Internet, a large majority of lakes are listed by anglers in articles or trip reports. Finding articles that talk about a particular season or fish are your best chance of finding out where and when to go fishing in a certain area or on a certain lake. You can also consult bathymetric charts on the Navionics Charts Viewer application, although the data is generally targeted at France's best-known large lakes.
You can also follow the actions of fishing federations and AAPPMAs to find out what's happening with a particular body of water or fish population. Each federation offers a small booklet listing all its lakes and rivers, and the fish they contain.