If all you need is "there or missing", the cheapest and simplest option is a microswitch under the bike stand to detect whether there is any weight in it. Wire the switch to a power source and light bulb or LED so it shows which have a bike in them and which are empty.
If you want to indicate when the switch changes state, in other words a bike is added or removed from the rack, you need a circuit that checks the switch state and remembers what is was, if a new check doesn't match the previous state, the bike has been moved. You can do this quite easily with an XOR logic gate, a resistor and a capacitor.
Brian.