It's rather easy to distinguish a page opened for editing, from the page, opened in read-only mode.

Look at the right side in the status bar: the page, opened as read-only contains a yellow label Read only.

You can quickly re-open for editing a page, opened only for viewing. On the File menu, point to Open, and then select Reopen R/O page for Editing.

See also

Force pages opening as read-only for some users