The command should output nothing in standard output, if xscreensaver is not running. It would work like light-locker-command --query, gnome-screensaver-command --query, cinnamon-screensaver-command --query and mate-screensaver-command --query.
ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: xscreensaver 5.15-3+deb7u1ubuntu0.1
ProcVersionSignature: Ubuntu 4.4.0-83.106~14.04.1-generic 4.4.70
Uname: Linux 4.4.0-83-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.24
Architecture: amd64
CurrentDesktop: XFCE
Date: Sun Jul 2 15:41:21 2017
EcryptfsInUse: Yes
InstallationDate: Installed on 2014-09-21 (1015 days ago)
InstallationMedia: Ubuntu-Studio 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.1)
SourcePackage: xscreensaver
UpgradeStatus: No upgrade log present (probably fresh install)
[Expired for xscreensaver (Ubuntu) because there has been no activity for 60 days.]