It is very easy. All you have to do is think logically
1st place to look is in the logs.
On Redhat, and variants, the file to examine is /var/log/cron I am not sure about its whereabouts on Debian, but I'd suspect it's pretty much the same.
To see the last few lines of that file.
That file is basically a list with the times and commands that cron executed.
Another option would be to add a line to your crontab like this
* * * * * wall test
If your cron works, then after two minutes, you will see the word test appear on your console every minute