"One possibility would be to look at oxygen isotopic abundances in the annual growth bands of teeth or bones of high latitude (and hence, presumably, seasonally influenced) terrestrial dinosaurs. The ratio of oxygen isotopes depends on temperature. An absence of seasonal variations in oxygen 18--a heavy version of the common oxygen 16 atom--would strongly suggest that the animals maintained a constant internal temperatures.
"Such a finding would not, however, constitute 'proof' that dinosaurs were warm blooded, as there are external mechanisms that cold-blooded animals employ to regulate body temperatures and thereby influence metabolic rates. Proof would have to come from discovery of intact dinosaur remains in which the soft tissue had not been replaced or altered, and from which the biomolecules responsible for thermoregulation could be extracted, identified and characterized. Such a proof is very, very unlikely, as it would require an almost impossible level of preservation over 65 million years, plus the advent of biotechnology that does not (yet) exist to elucidate the thermoregulation biochemistry of an extinct species.