sd_booted man page
sd_booted — Test whether the system is running the systemd init system
sd_booted() checks whether the system was booted up using the systemd init system.
On failure, this call returns a negative errno-style error code. If the system was booted up with systemd as init system, this call returns a positive return value, zero otherwise.
These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.
Internally, this function checks whether the directory /run/systemd/system/ exists. A simple check like this can also be implemented trivially in shell or any other language.
sd-daemon(3), systemd.directives(7), systemd.index(7), systemd-notify(1).