Probably for any of the oils you find for sale on Amazon, the company can be found and researched a bit. I ordered from Mountain Rose Herbs a few years ago, then visited the store in Eugene Oregon when I was out there last year. They have good quality and seem to be very customer oriented.
However, for me, Thieves is a little overpowering. I have some Young Living Thieves that I hesitate to use much because it is so strong. Different companies have their own mix of Thieves. I think the ingredients are kind of flexible, as many oils have the same qualities.