Pads should be replaced if they are worn out, contaminated
Generally, the pad material should measure at least 1mm thick. By stacking 3 business cards or 1 credit card you can create a makeshift feeler gauge that is approximately 1mm.
Contaminants such as brake fluids, oils, or grease can ruin your pads, but they’re not always easy to see. Contaminated pads may cause problems such as a squeal, extra effort at the lever to slow down, or a darkness on the rotor and pads that smear when rubbed with a finger.
Always look for the source of the contamination. Some common sources are overspray from aerosol lubricants