The quickest way, of course, is to look at the percentage of MAGA voters. The higher the percentage, the dumber the population. This is infallible: Trump loves the uneducated, and they love him back.
But there's another exercise you can do. If you're driving through an area, tune your radio to FM and scroll through 88.5 to 91.5. First, see if there's an NPR station. Then count the number of God Squad stations. (You'll be able to quickly identify the latter: some guy with a cowboy accent yelling.)
Portland: 2 NPR, 1 God Squad
Medford, OR: 0 NPR, 4 God Squad
Bakersfield, CA: 0 NPR, 6 God Squad
Multiple studies have confirmed that if God Squad stations outnumber NPR stations by 6 or more, then the average IQ of the local populace is 10 or lower. For reference, the IQ of an alarm clock is 3.
You can also buy a cup of coffee somewhere, give the cashier a $5 bill, and see how long it takes them to figure out how much change to give you. But that requires actually stopping.