I put a gauge from Academy Sports just above the bottom grill grate.
It's been dead on for temps.
(I tried moving a oven gauge around on the grate and got consistent readings)
When I use my upper grate I use an old turkey cooker thermometer dropped thru one of the exhaust holes in the top.
Close enough for Government work...
A UDS may be the most forgiving smoker you can use.
The moist environment gives a little leeway in temperature.