I've rarely had that problem. If you know the correct size the file should be then you shouldn't have that issue. If it's zipped then it'll say it's mac. If it's dmg or sit, then it "should" be for mac. I don't believe I've ever seen windows files packaged in SIT files. Not that there'll necessarily be any reports but you could always check Bitzi to see if there's any user feedback on such files before you downld them.