I use an IR gun every time I stop. I check tires and brake temps along with the rear end hub.
There is no set temp you will see, what you are looking for is a consistent wide variance between tires. Sunlight and wind direction will play a part. 
While I am checking temps I accomplish a couple things...good walk around inspection, check tow bar connections, good exercise, hoodlums in the parking area, etc. 
After you do it several times you will get a feel for what is good. 
I  have a IR gun that has a little red laser dot that you use to aim it. 
They are about $50 give or take. Well worth the investment because you can use them for testing other things...faucet water temp, etc.