You can actually perform the test yourself with a good quality refractometer. You should be able to purchase one through your local wholesaler. Some precautions on use. Get clean samples of each oil(small sample of mineral,poe and poe/min mix) place the samples and the instrument in a location with about 70 degree ambient and let all sit there for a while. Once everything is same temp you can see where your percentage is based on a graph provided with the instrument. The biggest complaint i've heard with these devices is when a guy pulls the instrument out of a freezing van in the middle of winter and tries to take a reading on hot oil fresh out of a sump...will never be accurate this way.
I use a combination of FTIR and metals (amount of sulfur) to work out the residual mineral oil concentration. Occasionally I use a refractometer as well if I can't get a good answer with the other testing. The FTIR can also help with three way mixtures such as MO / POE / AB commonly seen in supermarket racks.
The refactometer works OK for field testing, but you need to completely strip out the dissolved refrigerant to get a good answer. The lab refractometers give one more decimal place than the field hand held units, so can be more accurate.