How to know if I am using systemd on Linux?

Check what process is running as PID 1. You can do this by running ps 1 and scrolling to the top. If you have some systemd thing running as PID 1, you have systemd running.

Alternatively, run systemctl to list running systemd units.

You might also want to check what /sbin/init is; file /sbin/init will tell you if it's a real executable or if it's a symbolic link to some other package's executable. On a systemd box, for example:

root@boxy / # file /sbin/init
/sbin/init: symbolic link to ../lib/systemd/systemd

For more information, check this out: https://en.wikipedia.org/wiki/Linux_startup_process

Another way of seeing exactly what you have on your system is typing man init and seeing which program's man page you end up on.

I know this is an old question, but since I was just asking myself the same question - here are my 2ct.

Best solution I came up with

ps --no-headers -o comm 1

This returns either systemd or init and appears reliable across Linux distributions and releases.

file /sbin/init would work, with help of pattern matching. Output of ps 1 does not appear helpful since on some Linux distributions it will print 'init' (the symlink) despite systemd being used.

Debian 8

$ ps 1
  PID TTY      STAT   TIME COMMAND
    1 ?        Ss     0:02 /sbin/init
$ file /sbin/init
/sbin/init: symbolic link to /lib/systemd/systemd

RHEL 7

$ ps 1
  PID TTY      STAT   TIME COMMAND
    1 ?        Ss     7:46 /usr/lib/systemd/systemd --switched-root --system --deserialize 21
$ file /sbin/init
/sbin/init: symbolic link to `../lib/systemd/systemd'

SLES 12

$ ps 1
  PID TTY      STAT   TIME COMMAND
    1 ?        Ss     0:24 /usr/lib/systemd/systemd --switched-root --system --deserialize 21
$ file /sbin/init
/sbin/init: symbolic link to `../usr/lib/systemd/systemd'

openSUSE 13.1

$ ps 1
  PID TTY      STAT   TIME COMMAND
    1 ?        Ss     0:33 /sbin/init showopts
$ /sbin/init: symbolic link to `../usr/lib/systemd/systemd'

The correct solution is to check the presence of /run/systemd/system directory.

[[ -x /run/systemd/system ]] && echo "using systemd" 

This method is used by systemd's own library function sd_booted(): https://www.freedesktop.org/software/systemd/man/sd_booted.html