You can use
brew ls --versions myformula
to output the installed versions of the respective formula. If the formula is not installed, the output will be empty.
When using a recent versions of homebrew, which you can get with brew update, you can just run this (thanks Slaven):
if brew ls --versions myformula > /dev/null; then
# The package is installed
else
# The package is not installed
fi
That said, it is probably a good idea to check for the existence of the tool at all and not just checking for the respective homebrew package (e.g. by searching for the executable in the $PATH). People tend to install tools in a rather large amount of ways in practice, with homebrew being just one of them.