/usr/share/dnsmasq/installed-marker is in dnsmasq 2.76-5+deb9u2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | # This file indicates dnsmasq (and not just dnsmasq-base) is installed.
# It is an implementation detail of the dnsmasq init script.
|