Liquid Metal (LM) thermal interface materials (TIM) have almost the exact same composition and their claimed thermal conductivity is probably overstated by a factor of two. I did some spectroscopy on some of them 1-1/2 years ago.
LM is still the best thermal interface material you can get at the moment in terms of thermal conductivity and its ability to wick and spread thin, but every time I see or hear and ad for LM with a thermal conductivity called out it irks me to no end.
There is no agreed upon ISO/ANSI/ASME/ASTM standard for testing TIM but there are testing standards for the raw thermal conductivity of a material. These tests basically measure the thermal gradient across a known distance at the center of a sufficiently large surface. It being a liquid/paste makes it a bit more difficult in terms of containment, but not an insurmountable issue. While the amount of TIM required to perform this test is substantial for an individual, why LM TIM companies doesn't just do this is beyond me. Instead they rely on tests that try to measure the thermal conductivity with incredibly thin samples.
Then again, testing TIM by thermal conductivity alone is utter bull. At two interfaces, it is the contact resistance is what really that matters. Ideally, the TIM shouldn't even form a layer, only fill in the troughs in the surface roughness.
However, testing contact resistance gets way more complicated as variables such as particulate sizing in the TIM; surface roughness, hardness, and elasticity of the two interfaces; and contact pressure are all variables that would have to be accounted for.