My knowledge about brake pads come from the automotive world. Normally on the bikes that run disc and hydraulic, you have a two piston closed caliper and such a caliper has a service window on top. The same principle as a monoblock caliper on an automobile. You could see the brake pad wear through the service window as well as the longer travel of the brake lever, indicates the pad wear.
You can either choose organic or ceramic pads for your bike. Organics work well under all conditions and are also cheaper. Ceramic pads work amazing once reaching proper operating temperature and are a little bit pricey. If you run a trail with lots of hard turns and some high speed straights and sweeps, ceramic compound is your option. If you run a normal trail that is composed of medium to low elevation and skill, you bet is organic pads. Based on the riding that you normally do it dictates the type of pad you might want to run. Normally all stock bikes come with organic pads.
Hope that answered your question. 🙂