Autosave related settings of Webstorm 10.0.4 are mostly in [Settings -> Appearance & Behavior -> System Settings -> Synchronization].
By default the "Save files automatically ..." is unchecked. So Webstorm will not save file changes before you press Ctrl-S. But "Save files on frame deactivation" is checked. So if you switch to another application other than Webstorm, all changes unsaved will be saved to disk. However, when you modify file A without save, and switch to another tab to edit file B, file A will not be saved, because this is not "frame deactivation" (why?).
This is convenient for Meteor development. You can modify many files without save. As you switch to browser, Webstorm will save all changes to disk, then Meteor detects file changes and refreshes pages in browser.
In Webstorm 10.0.4 the Linux version, the blue color of changed tabs is subtle (compared with that on Windows or Mac), check "Mark modified tabs with asterisk" in [Settings -> Editor -> General -> Editor Tabs] to make unsaved tabs easier to identify.
Finally note that "Ctrl-S" in Webstorm is "save all" instead of "save".