The following command opens a file manager to show the directory.

dbus-send --session --dest=org.freedesktop.FileManager1 --print-reply \
 --type=method_call /org/freedesktop/FileManager1 \
 org.freedesktop.FileManager1.ShowItems array:string:"file:///path/to/your/directory" string:""

The problem is that the default file manager seems to be random. My guess is the last updated file manager, because it changed after a file manager was updated. Also, when a file manager is already running, it uses that file manager.

Instead of letting dbus-send determine which file manager to use, if I know which file managers are installed, can I set a specific file manager, like Nemo?