how to tell if a file is being downloaded by wget?

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http://unix.stackexchange.com – I use wget to download things(most are zip files) automatically for me during night. but sometimes at morning I find that a few files connot be unzipped. I don't know why this is happening, perhaps it's something wrong with the remote server. now I want to write a script to test zip files in my download folder periodically using 'unzip -t', but I don't want to test on the files that are being downloaded. So how can I tell if a file is being used by wget? (HowTos)