Warranty and guarantee are two different things. Customer doesn't have to prove anything, as the customer doesn't have to be tech-savvy. The manufacturer doesn't sell anything direct to customers, if at all as the Apple stores do, then the Apple store is the seller. The seller is obligated to receive the disputed device back, and send it to the repair service, even if it is maintained by the seller. The seller can't refuse to receive the non-working device, as Apple stores do in the US. If the service wants to prove that the damage was by the customer, the service has to prove that, and that's not easy. It is much nicer to live in the EU!