How to know where the coast is either flat or cliff?

You will need to learn to read topographical maps. They have lines representing changes in elevation.

When lines are very close together, those indicate rapid change in elevation. Lines far apart show flat areas, because the changes in elevation are much more gradual.

Cliffs would have many lines extremely close together.

I do not know if such maps have been made for areas covered by water.

The UK has been extensively mapped by the Ordnance Survey for many years and as @Hawaii_Jake says the contour lines on these maps will tell you how steep the coastline is. Ordnance Survey maps will also tell you whether the coast is sandy, muddy or rocky.

The Admiralty produces large-scale charts for main ports and harbours, medium scale charts for coastal navigation and small-scale charts for offshore navigation. It has distributers worldwide.

@Hawaii_Jake @flutherother Is there a website where I can check a map that only shows contour lines?

This type of map would be useful, but for your purposes you’d need a close-up to provide more detail. https://www.orangesmile.com/travelguide/afoto/ocean-maps.htm

As other jellies have said, a topography map will give you terrain elevations.

If you want to know for vacation purposes, because let’s say you want a wide sandy beach, the easiest way to research it is through travel websites and travel related social media groups. There are plenty of beaches that feel at sea level, but are several hundred feet up. Like beaches along the Great Lakes in the US. On parts of the shore at the Great Lakes the land does meet the water on level. Some of those beaches are sandy and some are full of rocks.

Sea level and land and sea at similar elevations is no guarantee of lots of smooth sand.

LIDAR (Light detection and ranging) mapping is currently used to map the Earths surface and underwater depths. It utilizes different color spectrums an can map down to 3 meters even from satellite.
Here is a good basic article

@yaujj48 “Is there a website where I can check a map that only shows contour lines?”

Go to maps.google.com and on the “layers” button, turn on the terrain. This will overlay topographical lines on the map.

It takes a bit more map reading than just elevation and contours. By only knowing elevation, one does not differentiate between a sandy beach and a marshy fen, or if the shoreline is full of rocks.

And in many places, the shoreline is beach at the foot of a cliff. So in this day and age, aerial photos are the best way to be sure. I lived here when I was in college, bluff cliff with beach at the bottom.

There’s also Google Earth. They’ve got satellite pictures of most of the planet.

You might Google beach towns in your area of interest, if that’s what you’re looking for, or coastline features in your area of interest. Also you might just look at coastal photos for the region.