check it by placing the tip in boiling water and it should read 212° +/- then set in icewater and look for 32°
If you're looking for smoker temp, insert the prob thru something like a tater and place on the grate you have you food on. you don't want it directly on the hot grate Edit: if it's a dial therm you might want to drill a hole in the lid and inset it thru the hole. some dial mometers can't take the heat
What dan ^^^^^^^^ said he gave you the exact way to test you tempometer. Then just place it in the smoker where you put your meat/food to tell what the smoker is cooking at. Don't forget the one for your meat to. thats why we say you need atleast 2 if not 3. But test them all.