The list includes...
1) Ask manufacturer if facility is dedicated to GF products. If not, ask if the production line is dedicated to GF products. If not, ask about their cleaning standards (I tend to shy away from eating things on shared lines no matter the cleaning standards). You can also check to see if they test their food regularly when it comes off the production line (using either R5 ELISA or the omega-gliadin ELISA). 20 PPM is the level of gluten you are looking for here.
2) Check the manufacturer's Certificate of Analysis which will show their ingredient suppliers also follow strict standards for identifying gluten. Same tests apply here (R5 ELISA and the omega-gliadin ELISA). Again, 20 PPM.
3) Check if the manufacturer has been certified by the GFCO.
It also has the link to the proposed FDA rule on Gluten Free food labeling which should be officially released in the next few months. It's an interesting read.