How can I tell if a downloaded ISO is complete?

The easiest way, assuming that the source provides the correct output for you, is to take a hash of the completed file. If the hashes match, you're good to go. In this specific example, I know Microsoft publishes the MD5 or SHA-1 hash of their major ISOs. (I believe most legitimate software vendors who provide ISO files have this readily available these days.)

EDIT: This information may not be readily available directly on the download page, but you can often find it if you look. This is really the only 100% foolproof way to be absolutely sure you have what you're supposed to.

A quick-n-dirty method: if you install 7-Zip, you can right-click an ISO and open it as an archive - something I usually do anyway to unpack it (as an alternative to burning to DVD). From there, you can test the archive using the 'Test' command in the toolbar.

The ISO from the download center is the same one you can get from MSDN.

The MD5 is 61c2088850185ede8e18001d1ef3e6d12daa5692 vs2010sp1dvd1.iso

I hope that helps. If you don't have a MD5, you can download one here.

http://corz.org/windows/software/checksum/simple-checksum.php