While you want them to have started with good liquid oil, it seems to be a function of how they treat their oil that matters as well. Do they change it often? What are they cooking in it? Lots of frozen stuff? (water). Meat?
You know good clean oil when you see/smell it. Clear, fresh, liquid. Shop around, you will find decent stuff.
Here is an good read from Neoteric on the subject:
http://www.biofuels.ca/page16.html