If you are using something like com.oreilly.servlet,
you should find that it will throw an Exception you can catch (most likely unexpected end of part)?
It's fairly difficult to separate out (especially with more than one file, or multiple form elements) exactly how much of the POST data is 'the file'.. but what you Might attempt is to retrieve that HTTP header that tells you the size of the POST, and if your file is signifigantly smaller than this, you know it got chopped off.
But there's got to be a better way, right? Hmm...