How can I tell which version of Varnish I'm running?

According to the varnish documentation, it's varnishd -V

varnishd -V

This will return you something like:

varnishd (varnish-4.1.5 revision 2c82b1c)
Copyright (c) 2006 Verdens Gang AS
Copyright (c) 2006-2015 Varnish Software AS