Various embodiments include an offline leak detection system for a turbomachine fuel system. In some embodiments, the leak detection system includes: a fluid supply system fluidly connected to at least one fuel line of the turbomachine, the fluid supply system for delivering a non-flammable fluid to the combustor; a control system operably connected to the fluid supply system, the control system controlling a flow of the non-flammable fluid through the at least one fuel line, and controlling a pressure of the non-flammable fluid in the at least one fuel line to a pressure substantially equal to an operational fuel pressure of the turbomachine; and an optical monitor for determining a presence of the non-flammable fluid on an exterior of the at least one fuel line, the presence of the non-flammable fluid on the exterior of the at least one fuel line indicating a leak.