Humanoids are approximately as warm as real-world humans; yuan-ti are somewhat ambiguous.
Wilderness Survival Guide (1986), p. 74, discusses the use of Infravision (the ability to see into the infrared spectrum in order to see in the dark) regarding humanoids and other creatures.
Humans are stated to have a body temperature of approximately 98 degrees (Fahrenheit, it must be assumed), which is equivalent to the real-world human typical body temperature of 98.6°F.
A human (body temperature approximately 98 degrees) wearing nothing thicker than normal clothing will appear brighter than his surroundings when teh air temperature is 90 or lower. The same holds true for demi-humans and most size M humanoids, assuming that they have body temperatures roughly the same as a human's.
Infravision will not detect the presence of cold-blooded creatures (reptiles, insects, and most other nonmammalian creatures) because the temperature of such a creature is, by definition, the same as that of the surrounding air.
What this means is that mammalian humanoids are most likely to have body temperature similar to humans, though I do not believe there is any precise measurement for non-humans in the lore; while reptilian creatures will often have lower body temperatures. Thus, using a thermometer to detect yuan-ti by body temperature is plausible if they are reptilian rather than mammals.
Yuan-ti are described as reptiles, though with purebloods it is sometimes ambigious.
In this case, the question comes down to whether yuan-ti purebloods are warm-blooded like mammals, or cold-blooded like reptiles. This isn't clearly specified in the lore.
Dragon Magazine #305, Venom and Coil: The Secret Life of the Yuan-ti states that the yuan-ti are essentially reptilian:
Although yuan-ti view all mammals, even sentient ones, as innately inferior creatures, they pay great homage to their fellow reptiles.
...as preposterous as it sounds, a few humans have traces of reptile blood coursing through their veins.
Yuan-ti purebloods are described in this article as looking essentially human, but possessing serpentine features upon closer inspection. Volo's Guide to Monsters describes purebloods as the least snakelike of the yuan-ti, with yuan-ti sometimes laying eggs (as reptiles), but sometimes giving live birth (as mammals).
- Precise core temperatures of humanoid races are not given, but they are believed to be close to that of humans.
- Yuan-ti are described as reptilian, which would make them cold-blooded; however, the purebloods are most humanlike, and have more mammalian characteristics, so it's not necessarily guaranteed that they would have a lower body temperature.