Check "man crontab" for the exact location of cron's files on your system.
Please state the unix/Linux version - there is much variation.

In general:
Check /usr/adm/cron/log to see if the job ran at all.
Run "fuser /usr/adm/cron/log" and check there is a process using the log (i.e. cron is running).
Check mail for the user of the cron for error messages.
Check /usr/adm/cron/cron.allow and /usr/adm/cron/cron.deny to make sure that the user is allowed to run a cron.