1. Launch Terminal from Applications / Utilities
2. In the command line enter "du -sh *"
This command calculates the disk space for all the directories in /Users/YOURUSERNAME/
Check in particular the numbers for the folders Documents, Downloads, Movies, Music, Pictures
To find out more about du, type "man du" in the command line of the Terminal window. The option -s gives the summary for each directory, -h reports the disk usage in "Human-readable" output, KB, MB, GB