What I've always done is ask them to send me a screenshot of their side deck before siding, then ask for another screenshot of their side after siding. Or if they forgot to take that second screenshot, they tell me which cards they sided.
That way, I know how good they are at picking the cards they need against a certain deck. But if they don't side at all because their main deck was good enough to annihilate me, it's fine. >.>