The radar shows red dots when enemies are visible to your teammates. If you see a dot disappear (enemy moves behind a wall), you can estimate their path.
Here are some tips to help you get the most out of the super simple wallhack CS 1.6 method:
Made walls transparent or semi-transparent so you could see player models through them.
Once the cheat is configured, activate it by pressing the designated hotkey (usually "F1" or "F2"). You should now be able to see through walls and other obstacles.
Some players swear by gl_zmax 0 and gl_monolights 1 (old OpenGL tweaks). While these aren't true wallhacks, they remove certain lightmaps and can make player models slightly visible through thin textures like smoke or glass.