Indications of a fuel leak may include, but not be limited to:
Abnormal decrease in fuel quantity;
If aircraft equipped with an FMC, you may get "Using RSV Fuel", or "Insufficient Fuel", or "Check FMC Fuel Quantity" message.
Fuel spraying from the engine strut or trailing edge of the wing, depending on where the fuel leak is.
Actions to be taken depend on your company's non normal procedures.
However, usually, common sense prevails, and landing at the nearest suitable airport is probably recommended.