To 100% diagnose a bad wheel bearing, you can follow these steps: 

Test drive the vehicle: Take the vehicle for a test drive, and pay attention to any unusual sounds or vibrations. If you hear a grinding or humming noise, it could be a sign of a bad wheel bearing. 

Check for play in the wheel: Raise the vehicle off the ground and check for any play in the wheel. Try to move the wheel in different directions, and if it moves excessively or makes a clunking sound, it could be a sign of a bad wheel bearing. 

Check for heat: After driving the vehicle, touch the wheel and hub assembly. If it's hot to the touch, it could be a sign of a bad wheel bearing. 

Use a stethoscope: Use a stethoscope or a long screwdriver to listen to the wheel bearing while the vehicle is running. Place the stethoscope or screwdriver on the hub assembly and listen for any unusual noises. If you hear a grinding or humming noise, it could be a sign of a bad wheel bearing. 

Disassemble the wheel bearing: If you've tried all the previous steps and are still uncertain, you can disassemble the wheel bearing to inspect it for damage. Look for signs of wear, pitting, or rust, and compare it to a new wheel bearing to determine if it needs to be replaced. 

By following these steps, you can 100% diagnose a bad wheel bearing and make the necessary repairs to ensure your vehicle is safe to drive.