- int sd_booted(void);
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.