Update 2013-Oct-4: Looks like this doesn't work anymore. Now Google Docs Presentation seems to retrieve image-by-URL once upon inserting, and therefore no web site tracking tools work anymore.
Update 2014-Jan-11: SEEMS to be working for Sheets but not Docs
Any web site analytics tool can help. For example, www.statcounter.com (it's free):
- Go to statcounter.com and register for a free account.
Under “Install Code”, choose the following options:
Invisible Counter -> Google Pages -> HTML only counter.
Statcounter will now generate some HTML code enclosed in a
<div> tag. Strip everything except the URL in the IMG tag, it looks like this:
Open the Google Docs document that you want to track.
- In Google Docs menu, choose:
Insert > Image > By URL > Paste an image URL here:. Insert the above URL.
Currently it works at least for Documents, Spreadsheets and Presentations.
This answer is based on Paul Rowland's answer and the blog post he linked to.