This is a long running and quite complicated debate in palaeontology. First it is important to understand what the terms 'warm blooded' and 'cold blooded' mean. The reactions occur most efficiently at a particular temperature. Cold blooded' animals take advantage of external heat by basking in the sun, for example.
Endothermic animals have a continually high requirement for food to fuel the heat generation.
Providing they can find food their constantly-optimum body temperature means that they can stay active even when it is very cold. There have been many attempts to find features preserved in fossils which may give a clue as to whether an animal was endothermic or ectothermic. None of these, however, has proved to be conclusive. The most suggestive is the fine structure of the bone. Although bone looks solid and dead, it is in fact a living tissue.