Infrared thermometer is the easiest to use. I usually place a small peice of black electrical tape as close to the LED as possible, and measure there with the IR thermometer as close as possible to the tape. Reason for the tape is to remove an emissivity issues with the shiny metal surfaces.
if you look through the datasheets it will tell you how to get the junction temp, you need soldering point temp, and the drive current, and ambient air temp actually also plays a big role in led life
