It is good practice to constantly monitor the amount of bandwidth used and also the amount of incoming and outgoing packets per second. As part of your planning process, be sure to reserve bandwidth on your peering port for events like popular sporting events, key holidays or other reasons for high peak traffic. Also keep in mind DDoS attacks do happen. If your port usage reaches 90% or more, DE-CIX will actively contact you regarding an upgrade.
A necessary tool on your side to verify traffic flow and to actively plan and monitor your traffic is a traffic monitoring tool (e.g. NetFlow, JFlow, sFlow, or simply monitoring your interface counters, depending on your equipment).
You should know which AS numbers you exchange traffic with and at what times. Second, you need to have one of these tools in place to be informed about the nature and origin/target of your traffic, in case of an incoming DDoS attack.
If you do not have a traffic analysis tool in place, please speak with your engineering team or with consultants, who can assist you in implementing one.