All of my temperature evaluations start with a 1/8 " tubular thermocouple probe adapted into the intake manifold as close as possible to the thermostat. Do be careful not to BURY the probe against the bottom of the opening/port as it will impact the readings and sensitivity. This will put the probe in direct contact with the coolant as it is leaving the motor. The main reason for using Thermocouples is they are extremely accurate, sensitive, don't lie, nor do they need calibration. The meters may but rarely do unless abused.

You can buy extension leads to run inside the car and watch the actual temperature changes. most Digital multi meters today have a plug in for the T/C's. Make sure you get the correct type of probe and extensions for your particular meter. Type J and K TC's & extensions appear to be, BUT are not interchangeable and will skew the readings. Both types are available inexpensively on eBay or Amazon as well as various meters / DVM's etc. An example is of the probe is in the link just below this paragraph. I'm sure there are others and possibly less expensive. I use an 1/8" oil pressure line compression fitting and adapt it up to what ever size is needed. The metal portion is also bendable to a point but not on a repetitive basis. If you are careful and do not overtighten the fitting you can move the ferrule or carefully remove it to allow repositioning for the next use.
THERMOCOUPLE EXAMPLE LINKY

You may be surprised at things that affect the temperature, like a semi changing lanes in front of you and the temp starts ticking upward until you fall a good distance behind him. Going north on I-29 can make the temp climb a bit, while turning around and going south makes it drop. This is on what appears to be level ground.
These scenarios have repeated many times with different vehicle's over the years.
As we all know, gauges can be wrong for many reasons, IR guns are good for a general indication but they need to be calibrated for the surfaces / coatings they are reading as well as the distance from those surfaces there are many variables that can affect their accuracy.
keep us posted