What are the indications that your turbofan engine has "thrown a blade?"
Turbofan engines are designed and tested to contain a fan blade failure. However, this is a violent event and will cause serious damage to the engine. The engine and surrounding components are subjected to large forces and vibrations, and the engine can ingest debris from the damage to the fan, causing further damage. There will likely be an investigation, and the engine will probably be scrapped due to the widespread damage to different components.

To the pilots, it will seem like a bird strike, but probably worse. There will be a loud noise as the fan blade separates and collides with the fan case and following blades. The fan will then be out of balance, so heavy vibrations will follow as the shaft loses its rotational energy. The pilots will be able to tell which engine has failed by the loss of thrust, as well as the indications of vibration and failure on the instruments for that engine.

If the fan does not get stuck, it will continue to windmill, which will result in ongoing vibration from that engine.