A simple trick is to use a small ball or marble and a basic spirit level. Place the ball in the middle of the room or at different spots and see if it rolls strongly in one direction. A gentle slow roll isn’t usually a big issue, but a fast slide suggests a noticeable slope.
For a more precise check, put a spirit level on top of a long straight object, like a sturdy piece of wood or a metal ruler, and lay it across the floor in various directions. If the bubble settles far off centre, you know there’s a slope or dip below.
When installing furniture like wardrobes or bookshelves, check level at the point where they’ll stand. Minor unevenness can usually be fixed with adjustable feet or small shims under the legs. You don’t have to chase a perfectly level floor everywhere, but it’s good to know where the worst spots are before placing heavy pieces.