which nginx
will give you the path of the nginx being used
EDIT (2017-Jan-18)
Thanks to Will Palmer's comment on this answer, I have added the following...
If you've installed nginx via a package manager such as HomeBrew...
which nginx
may not give you the EXACT path to the nginx being used. You can however find it using
realpath $(which nginx)
and as mentioned by @Daniel Li
you can get configuration of nginx via his method
alternatively you can use this:
nginx -V