There is great introduction here:
http://wikimatze.de/testing-helpers-in-padrino.html
However do not forget that helpers are a Sinatra feature, so you might want to search for "sinatra helper testing":
http://stackoverflow.com/a/8575684
Both links are the best way for your "spec/unit" tests. However once my own helpers evolve, I start depending on other helpers (Padrino::Helpers) and features/variables/extensions from my Padrino::Application.