A rocket stove can be called so because it has an L or J shaped combustion core, with a vertical riser to increase draft and allow combustion to complete. Small or metal ones may not have as good efficiency as bigger ones, but if the core design gives proper turbulence and dwell time, combustion will still be good. In any case, I don't think that a well-configured rocket stove would actually have more combustion in a surrounding barrel where the temperature is much lower.
When you get to the little tin can sized units, they can't have enough heat concentration or dwell time to achieve full combustion. It used to be that DIY designs for small units still emphasized insulation around the core; if ones are being sold with no insulation, I would hesitate to call them rocket stoves with or without a barrel.
There is one professional emissions testing unit I know of, the Testo 320 I believe, that Peter van den Berg uses for his stove development. It can give levels of oxygen, carbon dioxide, carbon monoxide, efficiency, temperature (usually of the exhaust to test how much heat is being lost in spent gases), and a filter that gives a relative gauge of particulate emission by how clean or dirty it looks at the end of a run. Unfortunately it costs a couple-few thousand dollars.
A core designed according to published proportions and sizes should have no volatiles left after leaving the riser. The only real way to test this, aside from gross differences in output, would be with a Testo. You might be able to get a decent relative approximation by actually sniffing the (cooled) exhaust; this would only be practical among relatively clean designs. I once made a firebrick mockup of a 4" batch box core, and could stand a few feet above the riser with my face fully in the exhaust and comfortably breathe. The flames in that little free-flowing core hardly reached more than halfway up the riser, but obviously burned clean.