Syncthing is one of the best tools for keeping folders synchronized across the internet. However, for one or the other reason, it occasionally stops to work on my Ubuntu 16.04 and because I use it in a "set it and forget it" fashion, I also forget how to check that it is operational.
Make it start automatically at boot time (as a system service): systemctl enable syncthing@username.service
Start the service: systemctl start syncthing@username.service
Check the status: systemctl status syncthing@username.service
The URL of the GUI:

However, the above solution does not work well if e.g. the files to syncronize reside on an encrypted folder which is mounted only after the user has logged in (e.g. in Ubuntu 16.04, there was the possibility to encrypt the user folder, which was only unlocked during user login). Alternatively, one can run the service as a "user service" like this:

cp /usr/lib/systemd/user/syncthing.service ~/.config/systemd/user/
systemctl --user enable syncthing.service
systemctl --user start syncthing.service

Syncthing seems to regularly loose a file called ".stfolder", which is supposed to be in the root of any shared folder (or it gets converted into a folder). For me the problem gets usually fixed by simply recreating that (empty) file (and deleting the folder with the same name if it exists).