you can do this already with default tools in most distros.
This is not a straight forward process but it is certainly possible.
Start a download...let it download 1-5MB (you don't need much for this)
Pause the download
Open your file manager (Ubuntu uses Nautilus) and navigate to the downloading file.
Lets say the file is "awesome-stuff-i-want.zip", you will also see a "download temp" file labeled as "awesome-stuff-i-want.zip_ug" or "awesome-stuff-i-want.zip_aria2" (if you use the aria2 plugin).
Make a copy of that file and paste it into the same folder which in Ubuntu results with "awesome-stuff-i-want (copy).zip_ug"
Rename that file to something like "awesome-stuff-i-want-peak.zip". It doesn't matter what you name it really as long as you remove the "_ug" or "_aria2" part but making sure you don't rename it the same as the original file as it will overwrite it and we don't want that.
Double Click the new "peak" file to open it in the "Archive Manager".
The archive (zip) will open up and display the contents of the file. You can't extract this of course as it doesn't have all the content but it does work to let you see into the zip before it is fully downloaded.
Once you are satisfied that this file is what you want, delete the peak file and go back to uGet and resume it to finish downloading.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
you can do this already with default tools in most distros.
This is not a straight forward process but it is certainly possible.