How can you detect that a disc brake system has worn brake pads?

You are correct that you can detect worn brake pads by the wear marks on the pads themselves and by a very low level of brake fluid in the reservoir. In German traffic safety regulations, brake pads have wear indicators that become visible when the pad material has worn down to a critical level. Additionally, as brake pads wear thinner, the brake caliper piston extends further, causing the brake fluid level to drop in the reservoir. These are clear signs that the brake pads need servicing. On the other hand, looseness in the handbrake lever does not reliably indicate disc brake pad wear, as it might indicate cable adjustment issues instead. Monitoring both wear marks and brake fluid level helps ensure safe braking performance.