League of Legends (LoL) Question: How to show viability from the guide?

What do you mean by "IT says statistics"?

If you want to show viability, a bunch of successive match history games on a gold or higher account should be enough. In any case, you can always accept feedback and "turn" your guide into a humour/non-serious guide if your build seems to be hard to pull off.

If you want to show that it's somewhat optimal you need to do numbers on damage or explain thoroughly how you'll make offtank Karthus work (it exists and it's a great guide), for example.

If you want some examples of good explanations for unbeleivable guides you can check any guide from Aqua Dragon. He's a former Master player (now Diamond 3) with a bunch of weird builds that actually work. He also explains quite efficiently why everyone of his off-meta builds work.