If you also have an ssh or telnet login to the remote site you could use the UNIX lsof command to see whether the file is open for writing by a specific process. For example, say your file is /tmp/test.iso, you could run

lsof /tmp/test.iso
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Which would give something like this:

COMMAND   PID USER   FD   TYPE DEVICE SIZE NODE NAME
isowrite 12533 root  3w   REG  254,1    0  235 /tmp/test.iso
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The FD entry shows you that the file is opened for writing by the isowrite process.

Not the most elegant method either (Corions solution is better, but requires that you have some control over the iso-creating process), but more reliable than checking for size change.