The answer to your original question is...........it depends. Pitch rate, gravity, pitch temp, strain have an influence.
A beer that is pitched and fermented above the optimal range for the yeast being used can take off quickly and vigorously. The overly-active fermentation at high temp can generate enough heat so to cause the wort temp to run as much as 8-10*F higher than the ambient air when it's really rocking.
Another beer that's pitched a few degrees below the optimal range and held at the lower end of that range will go through the process in a more controlled fashion and won't have as much heat being generated quickly by the fermentation. The exothermic reaction still happens, but it will be more spread out over time.
Temp probe taped to the outside of the fermenter and covered over with bubble wrap gives a reasonably accurate reading.