minitest.rb

require 'minitest/mock'

class Foo
  def foo
    bar
  end

  def bar

  end
end

f = Foo.new
mock = Minitest::Mock.new(f)
mock.expect(:bar, nil)
mock.foo

mock.verify
# Traceback (most recent call last):
#         5: from /Users/rschneeman/.rubies/ruby-2.5.1/bin/irb:11:in `<main>'
#         4: from (irb):18
#         3: from /Users/rschneeman/.gem/ruby/2.5.1/gems/minitest-5.11.3/lib/minitest/mock.rb:109:in `verify'
#         2: from /Users/rschneeman/.gem/ruby/2.5.1/gems/minitest-5.11.3/lib/minitest/mock.rb:109:in `each'
#         1: from /Users/rschneeman/.gem/ruby/2.5.1/gems/minitest-5.11.3/lib/minitest/mock.rb:111:in `block in verify'
# MockExpectationError (expected bar() => nil)