capistrano/docs/documentation/advanced-features/validation-of-variables/index.markdown

layout	title
default	Validation of variables

To validate a variable, each time before it is set, define a validation:

validate :some_key do |key, value|
  if value.length < 5
    raise Capistrano::ValidationError, "Length of #{key} is too short!"
  end
end

Multiple validations can be assigned to a single key. Validations will be executed in the order of registration.