To test LED lights on a bike you can use a small 9V battery (connected the right way around) to get the light to show dimly. If you don’t have one to hand, you may be able to borrow one from something, e.g. a smoke detector. Using this small battery is safer than connecting to the main bike battery as that can supply a very large current and has far more potential to do harm if accidentally poked in the wrong place. If you use thin solid core wire, this can poke into small connectors easily.
Note, it’s not possible to just use a multimeter to check for continuity as for LED lights that seems to just show open circuit, or very very high resistance value.
