brew ls --versions myformula

if brew ls --versions myformula > /dev/null; then
  # The package is installed
else
  # The package is not installed
end

for pkg in macvim ngrep other needed packages; do
    if brew list -1 | grep -q "^${pkg}$"; then
        echo "Package '$pkg' is installed"
    else
        echo "Package '$pkg' is not installed"
    fi
done