Is there a distinct look for contaminated disc brake pads?

I lost rear braking on by bike because mineral oil from the rear brake caliper got onto the brake pads. My symptom was a spongy brake handle response and reduced braking power. It seems that the caliper bolt lost a complete seal at the brass washer. I solved this by tightening the caliper bolt and sanding off contaminants from the problem pad and the opposite pad, for security.

I argue that braking is lost most often when air enters the brake lines and when brake fluid comes in contact with the pads. The remedy for the first is to bleed the brakes and to pinpoint the area of leakage. The remedy for the second is to stop the leak and replace or repair the pads.

Your situation may be similar to mine. But it could also be not.

I had the same problem with my Magura MT5 4-piston brakes. Since I am using sintered brake pads, I literally "torched the hell out of 'em".

I first soaked them in IPA (Rubbing alcohol >70% ABV). After placing them on a surface that won't be damaged by direct contact with fire, like a steel tray or even a brick, I lit the alcohol-soaked brake pads and watched as they burnt until the alcohol was totally burnt away.

Then using a medium-fine sandpaper (in this instance 240-grit) I sanded off the superficial layer of material leaving a lighter colored layer of braking surface. The oil has been totally burnt off. This will not work for organic/resin or semi-metallic brake pads. These variants do not deal with such extreme heat as well as sintered pads. It will ruin them.

I then cleaned the caliper with a pressure washer from a distance of about a foot-and-a-half, occasionally dousing with a highly concentrated Dawn solution like 50:50 water and soap. Only a small amount is needed (an ounce [30ml] or so). Using a fine pipe cleaner will help get any contaminants out of the caliper too. This prevents any contamination in the near future.

I then pressure washed my rotors too. After doing this, I soaked a coarse cotton cloth, like an old towel, in the alcohol solution and scrubbed my rotors free of any contaminants. It's amazing how much gunk is hiding invisibly on your rotors.

I then reassembled everything and bedded in my pads and discs. This is very important as you've removed all of the bedding from the pads and discs during the cleaning process. This is what I did to salvage my brakes from an, otherwise, dangerous contamination of my brakes. Hope it helps you.