How did Faraday, Ampere and Hippolyte Pixii know that electricity 'traveled' at all? Let alone in a certain direction? And that it was reversing direction with certain early generators?
Daniel Sank is correct; with a magnetized needle on a pivot and a coil of wire you can make a device called a galvanometer with which you can watch current change direction, at least for low frequencies of reversal. The early experimenters could connect galvanometers to their electrochemical experiments and correlate the direction of needle deflection to the direction of charge transport.
Benjamin Franklin, with his lightning kite experiment, was one of the first to realize that electricity travels.
Ampère knew that parallel wires with current going the same (opposite) direction attract (repel). Thus, if there an AC, you will see the wires vibrating like a plucked string.
In the 1830s many people were interested in knowing the "speed" of electricity in wires. Wheatsone designed a visual experiment to determine the speed of electricity the help of rotating mirrors. The original reference is Wheatstone, Charles (1834). "An Account of Some Experiments to Measure the Velocity of Electricity and the Duration of Electric Light". Philosophical Transactions of the Royal Society of London. 124: 583–591. Here you might get better and original references.