Per Amazing Ribs Thermometer Buying Guide:
The best method is a probe thermometer. Position the tip of the probe at the level where the food rests, about 1 inch above the grate or rack, and one inch from the food. You don't want it closer because of the cooling effect of the food itself.
You can suspend the probe above the grate with a clip if it comes with one. Otherwise, make a ball of aluminum foil, and simply poke the probe through it for an improvised offset stand.
Picture from Amazing ribs article linked above
Note: the referenced article is very long, and a wealth of detailed information. Search the page for the headline "I'll tell you where to stick it" (about half way down the page) and start reading there (okay, read the whole thing, but that is the section that is directly applicable to your question.)