This was such a problem for me I actually made a utility called "iCloudStatus" which basically behaves the way Dropbox does by showing a status icon in the menu bar (see below).
The other alternative is looking at the Finder but if you're uploading large batches of documents nested deeply in folders like I am then it will be a time consuming experience knowing when your files are uploaded.
You can download it
here and for full disclosure I am the developer.