Ask the vet - Rabbit
How can you tell if a rabbit has a fever?
Vet at FirstVet answers:
The only way to be certain your rabbit has a fever is by taking its rectal temperature. This is rarely done at home due to the stress induced from taking the temperature. Rabbits also have very sensitive mucous membranes in their anus which can accidentally be damaged by the thermometer.
Rabbits have a normal temperature of up to 40°C. It is only once the temperature exceeds 40°C that the rabbit is considered to have a fever. A common misunderstanding is that warm ears or a warm and dry nose are signs of a fever. This is not the case.
If you are concerned or worried that your rabbit is unwell you should contact a veterinarian as soon as possible since rabbits mask symptoms of disease for as long as they are physically able to.