Adsense for fallback ads (for testing purposes)

I'm switching to the latest "responsive" ads from Google, but I'm having a terrible time testing its fallback to html fallback ads.

I cannot find any way to trigger adsense to go for the fallback ads. They seem to have plenty of inventory, so even after letting it run for a few days on a high traffic page, I can barely get any fallback ads on real traffic. And none of those got delivered to my tests, nor that I could get them on different sized screens and devices to see if it actually even works as it should.

I even tried for a few minutes to add a page in a robots.txt excluded page and add some really bad words, but no luck: ads still appear. Not a hero to go further down that road ...

Anybody got a reliable way to trigger fallback ads on the newest code that's ToS safe ?