Inspect fan performance during operation to confirm fan is spinning. Check that vents are not blocked, and if necessary, use compressed air to remove dust or debris from rear fan exhaust.
Compare computer’s operating temperature to a known-good, similarly configured computer.
Check for runaway applications. Refer to Apple Support article TS1473: Runaway applications can shorten battery runtime. Follow instructions to halt any processes that are using excessive system resources.
Applications and system processes that are processor-intensive / graphics-intensive may cause bottom case to feel warm. Use Activity Monitor to identify these types of programs and explain issue to user.
Reset SMC using the procedure listed for this computer in Apple Support article HT3964: Intel- based Macs: Resetting the System Management Controller (SMC).