If you have five feet of 3/16" ID line that holds approx. a fifth of an ounce per foot, this would equate to about one ounce of beer in the line. So, to easily record the temperature (preferably when you are thirsty) simply draw a beer in a glass and then chug it. Or pour into another glass for a friend. Then pour a second beer into the initial glass to acclimate it to the temperature of the beer in the keg. Record the temperature of the beer in this glass with a calibrated thermometer. Does not get any easier than this.
Scott Zuhse, Instructor
Micro Matic Dispense Institute