First you can check the standard error from cron jobs, which is sent to system mail by default. If you have edited crontab as root, type simply mail
at the command line and see if any message has been sent from crond. Another hint is to put the full path of the commands in the crontab, since crontab itself has a very limited PATH by default, e.g.
00 8 * * 0-5 /sbin/ifup eth1
You can also test the crontab with a very simple command like
date > /root/cron.log
Also check if the cron daemon is running, e.g.
ps -u root | grep crond