How can I find available network interfaces?

The simplest method I know to list all of your interfaces is

ifconfig -a

EDIT

If you're on a system where that has been made obsolete, you can use

ip link show

The kernel lists them by name in /sys, both separately in (e.g.) the tree of PCI devices -- although finding them there if you don't know where they are to start with is not simple -- and together via symlinks in /sys/class/net. E.g.:

> ls /sys/class/net
em1 lo wlp6so

Another example:

> ls /sys/class/net
lo p6s1 wlan0

If you are not sure which is which, you could just put them all up with either:

ifconfig [name] up 

Or:

ip link set [name] up

And then look at the (fairly clear) clues listed by ifconfig (with no arguments). The systemd source code linked from the Freedesktop.org document also refers to:

Two character prefixes based on the type of interface:
 *   en -- ethernet
 *   sl -- serial line IP (slip)
 *   wl -- wlan
 *   ww -- wwan

Although in one of the cases above (p6s1) there is no prefix.

I always do cat /proc/net/dev it's simple and easy to memorize