I run my Mavic on 90° days and the motor temps after a 15 minute flight in sport mode are around 120°. If one of your motors is hotter than the others, maybe some debris got in there and is causing friction. If it's making more noise than the others, something might be wrong.
In any machine with multiple motors or cylinders, one motor or cylinder will always run hotter than all the others and one will run cooler. I have temped many a Mavic and have not noticed the LR motor getting any hotter than the others.
As others have suggested, get yourself a $9 Walmart temp gun and temp them before the flight and then after the flight. Temp the inside and the outside of the cans.