Add this line to crontab -e: */12 * * * * echo "testing" > /home/youruser/crontest
If you wait 10 minutes and see a file called crontest in your home directory then the problem is xfdesktop, maybe you need to call it differently.
Oh, and when you add that command, take a look as root (others users can't) at /var/spool/cron/crontabs/ there you should see a file that has your user name and if you open it it has the crontab commands (adding the command there won't work).
crond -l10 makes crontab to be more verbose when showing errors, you can see that in it's man page.
PS: Are you saving the file when using crontab -e right? I don't want to offend, sometimes it happens