Notes parameters to most recent call to Sinatra::Template.render and exposes them via #last_render.

sinatra_render_spy.rb

require 'sinatra'

module Sinatra
  module Templates
    @@last_render = {}

    alias_method :orig_render, :render
    def render(engine, data, options={}, locals={}, &block)
      @@last_render[:engine]  = engine
      @@last_render[:data]    = data
      @@last_render[:options] = options.clone
      @@last_render[:locals]  = locals.clone
      orig_render(engine, data, options, locals, &block)
    end

    def self.last_render
      @@last_render
    end
  end
end

def last_render
  Sinatra::Templates.last_render
end