Easy and simple, and recorded here for quick recollection:

$ brew --prefix openssl
/usr/local/opt/openssl

This is a symlink to the path in Cellar:

$ ls -l `brew --prefix openssl`
lrwxr-xr-x  1 dustin  staff  26 Apr 14 20:25 /usr/local/opt/openssl -> ../Cellar/openssl/1.0.2a-1