Use the billing alerts feature of the CloudWatch Alarms subsystem.
If you get an alert saying that you've hit fifty percent of your monthly spend-target before the 15th of the month, you know that something's gone awry. =)
Yes, this is a useful tool; but the Tool suggested in the next answer seems to be of more use. However, thanks for helping! :)
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.