The only way to review your previous true-up charges is to download a copy of your previous true-up bill.
To find and/or download a copy of your previous bills, sign in to Your Account. Look under the Your Account header and click the Bill & Payments History link to access the last six months. To access up to 24 months of bills and payments, use the scroll bar on the right-hand side of the module, and scroll down to the bottom to select the link for "View up to 24 months of activity" expander.
Note: you may need to click in the module for the scroll bar to appear.