How do I know if it's time to change my brakes? My pads all look like they have plenty of life, and my rotors don't have significant scratches.

I have only put 20k miles on the car in three years, and I'm not sure if I need to change the brakes and rotors.
There are several signs that can indicate it's time to change your brakes, even if the pads and rotors appear to be in good condition. These include:

Squeaking or squealing sounds: If you hear a high-pitched noise when you apply the brakes, it may be an indication that your brake pads are worn down and need to be replaced.

Vibration or pulsation: If you feel a vibration or pulsation in the brake pedal when you apply the brakes, it could be a sign that your rotors are warped and need to be replaced.

Longer stopping distances: If you notice that it's taking longer to stop your vehicle than it used to, it could be a sign that your brakes are wearing out and need to be replaced.

Reduced brake performance: If your brakes feel less responsive than they used to, or if you have to apply more pressure to the pedal to slow down or stop your vehicle, it could be an indication that your brakes are worn and need to be replaced.

Given that you've only put 20k miles on your car in three years and your pads and rotors look fine, it's possible that you may not need to replace them yet. However, if you're experiencing any of the signs listed above, it's best to have your brakes inspected by a mechanic to determine if they need to be replaced. It's always better to err on the side of caution when it comes to your brakes, as they are a crucial component of your vehicle's safety.
What are other useful things to check in my car at around 20k miles?
The maintenance required at 20,000 miles may vary depending on the make and model of your vehicle. However, here are some common maintenance tasks that are usually recommended around the 20,000-mile mark:

Oil and filter change: Changing the oil and filter is one of the most important maintenance tasks you can do for your vehicle. It helps keep your engine running smoothly and can prolong the life of your engine.

Tire rotation: Tire rotation involves moving the tires from one position to another on the vehicle. This helps ensure even tire wear and can extend the life of your tires.

Brake inspection: A brake inspection involves checking the brake pads, rotors, and calipers to make sure they are in good condition. This can help prevent brake problems and ensure your vehicle stops safely.

Cabin air filter replacement: The cabin air filter helps filter the air that comes into the interior of your vehicle. Replacing it regularly can help ensure good air quality inside your vehicle.

Fluid checks: Checking the fluid levels in your vehicle, including coolant, transmission fluid, and brake fluid, can help ensure they are at the appropriate levels and prevent potential problems.

It's always a good idea to check your vehicle's owner's manual for specific maintenance recommendations for your make and model. And, as always, regular maintenance and inspection can help prevent bigger, more costly problems down the road.