As long as it is ok to forcibly log-out the user, you just need to to logon as the local admin using
Apple Remote Desktop (available in the App Store) or ssh (through a terminal), and kill that user's
loginwindow process. You must login to the computer and not simply "Share screen" from Finder. Then you will be able to login.
Here's a really easy-to-read article about this.
Of course, for either of those options to work, you have to enable Remote Login and Remote Management in "Sharing" under the System Preferences. The Sharing menu is shown below:
If you would rather use fast-user-switching, which I personally don't like because it can often really slow down your computer, then you can set up an "logout user after x minutes of activity" option in the System Preferences -> Security & Privacy, which will return you to a login screen. There may also be nice hacks if you search around, that allow you to get the user-switching icon or a keyboard shortcut setup to do a fast-switch, or trigger a switch or logout by tweaking a call to CGSession, although that may have been possible only in older OSX versions. There is a huge discussion of many other ideas in that last link as well.