I burned some green bag lignetics before I got into the pellet cooker things and they worked well in my heater. Not sure if they still have different pellets or not, I know there used to be Blue bag ones also, not sure of the difference.
See if the bag has a place they are made, as I believe they used to have several facilities. Call and speak with the people there and ask what wood is used and where it is sourced and how the dies are lubed if needed, most seem to use a vegetable oil, if you are okay with the answers and try a bag, watch them burn in the burn pot a bit and look for the flame, I think you would be able to see impurities as they burn.
I use Hamer's Hot ones in my cookers with good results, they are an oak pellet.