First of all you need to check your existing crontab
This section discusses how to see if cron is currently running and to verify whether it's set up properly.
To verify whether or not your crontab is set up:
Log in to your Magento server as, or switch to, the Magento file system owner.
See if the following file exists:
1 - $ ls -al <magento_root>/var/.setup_cronjob_status
If the file exists, cron has run successfully in the past. If the file does not exist, either you haven't yet installed Magento or cron isn't running. In either case, continue with the next step.
Get more detail about cron.
As a user with root privileges, enter the following command:
2 - $ crontab -u <Magento file system owner name> -l
For example, on CentOS
3 - $ crontab -u magento_user -l
If no crontab has been set up for the user, the following message displays:
no crontab for magento_user
Your crontab tells you the following:
What PHP binary you're using (in some cases, you have more than one)
What Magento cron scripts you're running.
