As a parent to a toddler I was surprised to note how bad I am at estimating whether me or my child have a fever.

On top of that infants naturally can have a body temperature of up to 37.5°C, which is an additional hurdle because they may feel warm, but be generally fine.

Years of soldering taught me that the lips are very sensitive to temperature(much more so than fingers) and heal fast should you touch something really hot, like a soldering iron, so for emergencies there's that, but obviously it doesn't work when both of you have a temperature.

Also, for more than one reason, you can't reliably check yourself.