You can verify the integrity of downloaded files against checksums provided in SHA256SUMS or SHA512SUMS files on Debian mirrors. You can find them in the same places as the installation images itself. Visit the following locations: checksum files for CD images, checksum files for DVD images, checksum files for BD images, checksum files for other installation files. To compute the checksum of a downloaded installation file, use sha256sum filename.iso respective sha512sum filename.iso and then compare the shown checksum against the corresponding one in the SHA256SUMS respective SHA512SUMS file. The Debian CD FAQ has more useful information on this topic (such as the script check_debian_iso, to semi-automate above procedure), as well as instructions, how to verify the integrity of the above checksum files themselves.