The DFP Change History Tool allows you to see when changes were made and who made them. This tool is a little bit buried in the interface, but can be accessed under the Admin tab.
On the Change History screen, you can see who made each change and when it was made. The information can be overwhelming, especially on large sites with multiple traffickers, since absolutely everything is tracked. But there are a few filtering options that make it easier to use.
If there are multiple people on your team, the Change History tool is definitely worth checking out.