$ ls /usr/local/Cellar/curl/
OR
$ brew info curl
Sample outputs:

curl: stable 7.51.0 (bottled) [keg-only]
Get a file from an HTTP, HTTPS or FTP server
https://curl.haxx.se/
/usr/local/Cellar/curl/7.50.3 (367 files, 2.6M)
  Built from source on 2016-11-02 at 03:13:19 with: --with-nghttp2
/usr/local/Cellar/curl/7.51.0 (368 files, 2.7M)
  Built from source on 2016-11-08 at 01:10:09 with: --with-nghttp2
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/curl.rb
==> Dependencies
Build: pkg-config ?
Optional: openssl ?, libidn ?, rtmpdump ?, libssh2 ?, c-ares ?, libmetalink ?, libressl ?, nghttp2 ?
==> Options
--with-c-ares
	Build with C-Ares async DNS support
--with-gssapi
	Build with GSSAPI/Kerberos authentication support.
--with-libidn
	Build with support for Internationalized Domain Names
--with-libmetalink
	Build with libmetalink support.
--with-libressl
	Build with LibreSSL instead of Secure Transport or OpenSSL
--with-libssh2
	Build with scp and sftp support
--with-nghttp2
	Build with HTTP/2 support (requires OpenSSL or LibreSSL)
--with-openssl
	Build with OpenSSL instead of Secure Transport
--with-rtmpdump
	Build with RTMP support
==> Caveats
This formula is keg-only, which means it was not symlinked into /usr/local.
 
macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.
 
Generally there are no consequences of this for you. If you build your
own software and it requires this formula, you'll need to add to your
build variables:
 
    LDFLAGS:  -L/usr/local/opt/curl/lib
    CPPFLAGS: -I/usr/local/opt/curl/include
    PKG_CONFIG_PATH: /usr/local/opt/curl/lib/pkgconfig

Say hello to the cleanup option

The cleanup option can remove all installed or specific formulae, remove any older versions from the cellar. Also, old downloads from the Homebrew download-cache are deleted. Simply type the following command to free up disk space and delete outdated older versions:
$ brew cleanup
Sample outputs:

Removing: /usr/local/Cellar/android-sdk/24.2... (1,460 files, 105.3M)
Removing: /usr/local/Cellar/android-sdk/24.4... (1,471 files, 136.4M)
Removing: /usr/local/Cellar/atk/2.20.0... (205 files, 3.1M)
Removing: /usr/local/Cellar/babl/0.1.16... (37 files, 819.1K)
Removing: /usr/local/Cellar/bash/4.3.42... (59 files, 7.3M)
Removing: /usr/local/Cellar/bash/4.3.46... (59 files, 7.3M)
Removing: /usr/local/Cellar/bash/4.4... (145 files, 8.8M)
...
..
...
Removing: /Users/veryv/Library/Logs/Homebrew/libtorrent... (1.9K)
Removing: /Users/veryv/Library/Logs/Homebrew/most... (5 files, 124.5K)
Removing: /Users/veryv/Library/Logs/Homebrew/mtr... (68B)
Removing: /Users/veryv/Library/Logs/Homebrew/nmap... (6 files, 1.6M)
Removing: /Users/veryv/Library/Logs/Homebrew/openssl... (68B)
Removing: /Users/veryv/Library/Logs/Homebrew/rsnapshot... (3 files, 10.2K)
Removing: /Users/veryv/Library/Logs/Homebrew/ssh-copy-id... (68B)
Removing: /Users/veryv/Library/Logs/Homebrew/wakeonlan... (2 files, 470B)
==> This operation has freed approximately 3.9G of disk space.

A note about cask

Type the following to cleanup cask too:
$ brew cask cleanup
Sample outputs:

==> Removing cached downloads
/Users/veryv/Library/Caches/Homebrew/Cask/gimp--2.8.18-x86_64.dmg
/Users/veryv/Library/Caches/Homebrew/Cask/lisanet-gimp--2.8.14p2.dmg
/Users/veryv/Library/Caches/Homebrew/Cask/wireshark--2.2.3.dmg
/Users/veryv/Library/Caches/Homebrew/Cask/wkhtmltopdf--0.12.4.pkg
==> This operation has freed approximately 199.8M of disk space.