How do you know which channel is actually working?

Switch one off. Every dashboard credits itself, and the only uncontested measurement is what happens to total orders when a channel stops spending.

Why the answer is what it is

Run it for two weeks or a full purchase cycle, whichever is longer, and watch total revenue rather than that channel’s reported conversions. A channel that can be paused without a dip was reporting sales somebody else produced.

What this looks like in practice

The cheapest customer is the one you already have, and most budgets are spent as though that were untrue.

A channel that works at small spend often stops working at large spend, because the cheap audience runs out first.

Paid traffic is rented and gets more expensive every year. Every channel you own — email, notifications, your own audience — gets cheaper as it grows.

Where a mobile app comes into it

Push notifications sit on the owned side of that line, which is the whole commercial argument for an app.