Sorry for the delay... Just to make sure, what do you mean exactly by "nothing happens on the output"? If you're talking about the output section when you hover over the combinator, then that's expected to be empty. I can't just make the entity output whatever I want, so I instead use a hidden constant combinator that's connected to the outputs. This is functionally the same thing, but it won't be shown in the ui.
As for checking the output from lua: The only place the output is ever stored is the aforementioned constant combinator, so there is no real reason to even try it, since you can just hook the combinator up to a pole and see the exact output there. If you really want to debug it, the best way is probably to just dig into the code and log whatever you're interested in. The relevant files for this would be script/rc.lua and script/recipe-selector.lua.
