How do you recognize and prevent a hot tub rash (dermatitis / folliculitis)?

A clean tub. Make sure that the tub water is properly chlorinated if you are using a public tub and be sure to take a shower after.
Red bumps. Lots of little red bumps in the areas exposed to the hot tub water (ie: trunk, arms, legs). Showering after contact does not help prevent infection. Things that help are: continuous water filtration, frequent monitoring of disinfectant levels, frequent changing of water. The hot tub should be tested and treated by professionals for bacterial overgrowth.