There are a couple ways to tell if you're working offline.
In your Docs list, you'll see a status indicator in the upper right corner. When working offline, this indicator will turn gray.

From your doc, you'll also see a message in yellow at the top of your word processing documents indicating that you're working offline.

Note that if the offline indicator turns red, and the arrow inside is replaced by an exclamation point, this means that offline editing isn't working. In this case, to be safe, discontinue editing and copy your changes to a text file. Don't close your browser or the window in which your document is open. Instead, wait until you're back online to make sure your changes are saved.