How can I tell if a file is completely downloaded?

Hello, I had a problem which the above question partially addresses ('partially', perhaps pun-intended!). I downloaded some MP3 files, and found that only one of them downloaded fully, but two of them were only partial (part of the file). The problem is that they all play fine - just two get cut off at some point (they just stop playing) - and the time of the file audio length shows in Windows just the  amount that actually got downloaded (so everything looks normal). My question is that I have never heard of such a thing; in Windows, if a file copies over, it is either the whole file, or, if you click 'cancel' (or the power goes out etc.), then you have nothing; there is no such thing as copying over part of a file. If you wanted only part of an MP3 file, you would have to have an audio editor to cut it, and I didn't use anything like that for these files; I simply used them straight out of the box (the Dropbox, that is!). Also, they were files for making CDs. Now they've made about 200 copies and only realized what was going at this point. Because, although we had heard that it got cut of in middle, they figured that that was what had been sent, and just wasn't so perfect on the producers part. Now we realized this, but it hadn't entered my mind for a second to check if the whole file had downloaded because I have never heard of such a thing. (You can, of course, pause a download, and then it will be a 'partial download'. However, you will not be able to use the file until you have the whole thing). TY