Toyota PU possibly running hot

Hello, my problem is that when I first start my 1993 Toyota PU 4 cyl (22re w/165000) the temperature gauge almost goes into the red after about 7 minutes. Then it goes down about halfway and up again though not as much. It does this a little more and finally settles just below half. I have owned this truck for 7 yrs in the same location and it used to not do this. Just changed the coolant and put in a new thermostat and plugs (old ones looked ok if a little white) but nothing changed. I would like to check the actual temperature of the engine before the thermostat seems to open or just when it opens and have heard of putting some sort of temperature gauge in the radiator (where can I get one?) Or can I use a temperature gun? Any tips would be appreciated -ThankYou- Mark