Summarize this article in one sentence.
If you look in between the holes in your tire you can see the rotor, which is the round metal part that your rubber wheel fits over. Many vehicles will have drum brakes on the rear wheels, which have brake shoes rather than brake pads. Find the long metal piece pressing up against the rotor. The large metal piece clamped to the side of the rotor is called the brake caliper. If you look inside of the caliper you should see a rubber lining. This rubber lining is your brake pads.  This method is less accurate than taking your wheel off and measuring the brake pads. Make sure your car has been turned off for a while or it may still be hot. Shove a straw in between the caliper and the rotor. Continue pushing the straw until the straw hits the brake disc and comes to a stop. A vernier caliper is a measuring tool that's able to measure in small spaces where a ruler can't reach. Stick the end of the vernier caliper through the hole and read the top of the tool to get a measurement for your brake pads. You can purchase a vernier caliper at a hardware or automotive store or online. Use a marker to make a line where the straw and brake pad meet. Use a ruler or tape measure to measure the space between the end of the straw and your line. This will give you an approximation for how thick your brake pads are. The brake pad backing plate is about 5 millimetres (0.20 in)-thick, so you must subtract it from your number to get the accurate width of the brake pads. Your brake pads should be at least 1⁄3 inch (8.5 mm)-thick after you subtract 5 millimetres (0.20 in). if they are under 1⁄4 inch (6.4 mm)-thick. A new brake pad is usually around 1⁄2 inch (13 mm)-thick. Once it has worn half-way, you have to replace it soon. Brake pads that are 1⁄8 inch (3.2 mm)-thick should be immediately replaced and aren't safe to drive on.

Summary:
Look in between the spokes and locate the rotor on the front brakes. Find the caliper next to the rotor. Push a straw in between the brake caliper and rotor. Use a vernier caliper to get a more accurate reading. Make a line on the straw with a pen and measure it. Subtract 5 millimetres (0.20 in) from the number. Replace your brake pads