A cavity wall is like two thin walls in parallel, with a gap between them. This gap is the cavity, which helps to stop moisture coming in through the walls. Houses with cavity walls typically date from the 1920s onwards. If your house is older, it’s likely to have solid walls.
You can tell if your house has cavity walls by looking at the bricks from the outside. If they’re all laid with the long side facing outwards, that should be a cavity wall. If some of the bricks have the short side facing outwards, then it’s a solid wall.
If the brickwork has been covered, you can also tell by measuring the width of the wall. Examine a window or door on one of your external walls.
If a brick wall is more than 260mm thick then it's probably a cavity wall.
A narrower wall is probably a solid wall.