is_uploaded_file() emulation?

Hello all! How do I find out if a file was actually uploaded /without/ using is_uploaded_file()? My first though is that I should use fileowner() on the file and see if it's the same as the user who runs PHP (Apache) - but how do I find that out? I don't want to use exec("id -u") either because the syntax may be different for distinct systems and I'd like to avoid system calls if possible. I'm open to any suggestions to solve the original problem - not necessarily using UID's. Thanks! Bogdan