Generate an md5 checksum of [filename]:
md5sum [filename]
You can compare the output to the md5 checksum you downloaded with [filename] or you can use the -c flag to have md5sum do the comparison for you:
md5sum -c [checksum_filename]
E.g:
whoever@wherever ~ $ md5sum -c portage-latest.tar.bz2.md5sum portage-latest.tar.bz2: OK
If you have a digests file, this will contain the checksums for a number of files. E.g. Here's one from the gentoo livedvd-amd64-multilib-10.1.iso.DIGESTS:
# MD5 HASH fc94c887efea405b7aff6b62ab10e12a livedvd-amd64-multilib-10.1.iso dc311f02c98ce99f4aa860c8e30f5910 livedvd-amd64-multilib-10.1.iso.CONTENTS f4fa931a5637a55d58d76db788dbf46e livedvd-amd64-multilib-10.1.iso.CONTENTS-squashfs.gz # SHA1 HASH e6d0ddd45a0ae280deae6de43c548cc31496a9a0 livedvd-amd64-multilib-10.1.iso 255c08c0595d3498061434278166e20320f0b0a3 livedvd-amd64-multilib-10.1.iso.CONTENTS 49bea82820ec53aa5da0aa2f797371dce1c4738d livedvd-amd64-multilib-10.1.iso.CONTENTS-squashfs.gz
To automatically check all files listed in this digest, use the md5sum -c flag:
md5sum -c [digests_filename]
E.g:
whoever@wherever ~ $ md5sum -c livedvd-amd64-multilib-10.1.iso.DIGESTS livedvd-amd64-multilib-10.1.iso: OK livedvd-amd64-multilib-10.1.iso.CONTENTS: OK livedvd-amd64-multilib-10.1.iso.CONTENTS-squashfs.gz: OK
If any of the files listed in the digest are missing you'll get a warning message.