Since 1469-3 you can select the checksum plugin to be installed. This plugin will calculate the SHA-256 checksum of files sent/received via DCC.
The source code is available in the repo.
Note: calculating the checksum of large files can take quite some time, and during the process the XChat window will become unresponsive.
You don't need to download this for XChat-WDK, it's included in the installer. These packages are for other XChat builds.