I think (never used it myself) that there is a number that either the host (or the GWX team?) provide and it is a reference to the file content (size?).
You then run the MD5 prog to see if it has the same number once you've downloaded it - it should mean it's all there. Otherwise you've got another mighty download to do
I'm quite certain that somebody will correct us on this if it is in any way wrong - and so they should
Did you download from Jim's FileFront page, as the others have had problems recently?