If you have brakes that are made by any common and trusted brands, Shimano, SRAM, Campagnolo and the various other bicycle component makers that have been around for quite a while. Then you should be able to to find a model number on it and then go to that manufacturer's website and see what pads it sell and recommends for that model brake. Then if you don't want to buy their pads, you can look for other vendors that sell pads compatible for that brake model.

If you are having to wonder if the third party vendor brake pads have enough pad material or visually look to see if they match, then I'd probably suggest you find a better place to buy your brake pads from.