|the easiest and fastest way is compare the file's date, and if newer, do a compare.|
otherwise, you have to download the url/file and compare it to your last benchmark.
either way, no big deal, except for time-constraints/frustrations on very large files, and only if you need an exact compare. (also, md5 checksum can be used to avoid download-compare time).
Maybe if you can provide more details about what you need to do, and filesizes, it can be narrowed down to the best solution.