I don't own a bicycle. Whenever I go cycling, I would need to rent one. The first thing that I always check is to see if the brakes are working properly.
Sometimes, I have no idea that the gear is faulty until I encounter a steep slope where I need to paddle hard. This may cause the gear to jump or slip if it is faulty. Other times, when I change the gear, the paddling suddenly becomes inefficient, with constant grinding noises from the gear.
In the end, I try not to change the gears at all to avoid the above problems. I would like to know if there is a way to visually inspect the gear and tell the shop assistant that, "Hey, the gear is faulty!", instead of coming back halfway to do the exchange.