By the time your brakes are squealing, you've likely already worn through the pad material and started grinding metal backing plate against your rotor. That's not a warning sign anymore, that's damage already done. Here's how to catch it earlier.
Check Pad Thickness Without Removing the Wheel
Most calipers let you see the pads from the side without pulling anything apart. Look through the caliper opening at the pad material pressed against the rotor. You want to see at least 2mm of friction material left on each pad, not counting the metal backing plate. If you can barely tell where the pad ends and the backing plate begins, it's time to replace them, not next month, now.
If you can't get a clear view, a flashlight and looking from a slight angle usually does the trick. Don't just glance at the outer pad either, the inner pad on some calipers wears faster and isn't visible without checking both sides.
Inspect the Rotor, Not Just the Pads
Run your finger along the rotor surface, both sides, feeling for a lip or ridge at the outer edge. That ridge means the rotor has worn down and developed a step where the pads have been riding. A rotor with a noticeable lip is past its service life and needs replacing regardless of how it looks otherwise.
Also look for deep grooves, scoring, or bluish discoloration from overheating. Light surface scratches are normal. Deep grooves you can catch a fingernail on, or blue heat spots, mean the rotor has been compromised and won't provide consistent stopping power, especially under hard braking.
Feel the Lever and Pedal, Not Just Look
A spongy brake lever or pedal that sinks toward the bar or floor before engaging usually means air in the brake line or worn out brake fluid that's absorbed moisture over time. Brake fluid is hygroscopic, meaning it pulls moisture from the air over time even in a sealed system, and that moisture lowers the boiling point of the fluid. Under hard or repeated braking, that moisture can boil and turn to vapor, which compresses instead of transferring force, and your brakes go soft right when you need them most.
Most manufacturers call for a brake fluid flush every one to two years regardless of mileage, specifically because of this moisture absorption issue. If you don't remember the last time yours was changed, it's overdue.
Check Fluid Level and Color
Fresh brake fluid is usually a light amber or clear color. If yours looks dark brown or black through the sight glass on the reservoir, that's contaminated fluid that needs to be flushed. Also check the fluid level itself. As pads wear down, the caliper pistons extend further and fluid level drops naturally, so a low reservoir can actually be a secondary sign your pads are getting thin, not just a fluid problem by itself.
Listen and Feel While Riding
A grinding sound under braking means the pads are gone and metal is contacting the rotor directly. A pulsing sensation in the lever or pedal during braking usually means a warped rotor. A brake that feels fine when cold but fades after several hard stops in a row usually points to fluid that's absorbed too much moisture and is boiling under heat. Each of these is a different problem with a different fix, but all of them mean stop riding on it and get it looked at.
Bottom Line
Check pad thickness and rotor condition visually every month or two, and pay attention to how the lever and pedal feel every time you ride. Brakes are the one system on your bike where waiting for an obvious symptom means you've already been riding on reduced stopping power for a while. If anything here sounds like what you're dealing with, get it checked before your next ride, not after.