You do not always have to pull a wheel to get a rough read on your brakes. Through the spokes of many wheels you can check pad thickness and spot obvious problems in a couple of minutes. Here is how to do a no-wheel-off brake check.
Look at the pad through the spokes
On wheels with open spokes, you can usually see the brake caliper and the edge of the pad against the rotor. Judge the remaining friction material: a healthy pad has a thick block of material; if it looks paper-thin against the rotor, it is due for replacement - see minimum pad thickness. A flashlight helps.
Check the rotor surface
Look at the visible part of the rotor face. A smooth, even surface is good; deep grooves or scoring you can see means wear - see rotor scoring. A large raised lip on the outer edge indicates a worn rotor. Heavy bluing suggests overheating.
Use your ears and feel too
A visual check pairs well with the driving signs: squealing (the wear indicator), grinding, a pulsation, or a soft pedal all tell you something even without pulling a wheel - part of a simple monthly brake check. Together they give a solid picture.
When to pull the wheel
The through-the-spokes look is a screening check, not a full inspection - solid-spoke wheels hide the brakes, and you cannot see the inner pad or measure precisely. If anything looks marginal, pull the wheel for a proper look and measurement. When pads are due, fit quality pads and fresh hardware.