Is there a distinct look for contaminated disc brake pads? I have sintered pads and I'm trying as best as I can to diagnose a braking issue. I have Shimano XT M8000 callipers and the rear brake is being something of a pain.

If I leave the bike for about 4 days I lose almost all braking power. As I try and figure this out it would be good to know if there's anything distinct I can see or do to figure out whether ultimately the pads have been compromised.

This brake system uses mineral oil so I'd be looking for tell-tale signs of that or other lubricants. Is road grime a possibility too I wonder?

This issue is driving me crazy and I'm trying to find out if there are any leaks in the system through various methods.

Edit
My issue seems to be akin to the one discussed on this thread. http://forums.mtbr.com/brake-time/xt-xtr-brakes-get-squeal-power-loss-after-sitting-idle-6-8-weeks-902792.html 

My bike loses braking effectiveness when it sits idle. Quite an odd thing as I'd expect little to no pressure in the system to cause much leaking and surely something should be visible if it is leaking.

Edit 2 to document some fix attempts and diagnosis lines I've been down.

Firstly I manage to get power back by cleaning the pads with IPA and heating them with a gas flame as is mentioned in the comments below.

I've had the bike in the work stand and had a good look for any bubbles around the pots both with a brake bleed block in and with nothing (even pads) in. Bone dry.

I've wiped inside the calliper to see if anywhere is still damp to no avail.

I've visually checked the crimping around the hose and the banjo bolt but there's nothing that indicates build up from sticking to moisture.

Until today I was testing this with just one set of pads. Now there's a chance that my fixes are only good as a temporary measure whilst the bike is being used daily. After sitting maybe some of the contaminants are coming to the surface again. It could also be a build-up of silt from the ride along the canal towpath but I would have expected that to affect me overnight too and not after 3-4 days.

I took the pads out and let them dry for the whole work day one day this week and wiped them with a dry tissue and this did nothing to fix the symptoms.

I changed the rotors front and back although this only affects the rear brake.

I video'd the pots moving with no pads in just in case I could see something when Ina big screen. Not found anything so far. Might need to try it again with a bleed block in for resistance.

I'll add more if I think of anything else I've done.
I had the same problem with my Magura MT5 4-piston brakes. Since I am using sintered brake pads, I literally &quot;torched the hell out of 'em&quot;.
I first soaked them in IPA (Rubbing alcohol &gt;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.