I always use an Infra-Red Temperature Gun
http://www.ebay.co.uk/itm/Handheld-Non-Contact-IR-Laser-Infrared-Digital-Temperature-Gun-Thermometer-/172527792934?hash=item282b753326:g:uvYAAOSw4CFYn~2I
Measure all around the engine for temperature hotspots, etc.
Also useful to measure misfires, brake efficiency/balance, and a whole host of other useful stuff