When you screen the clouds turn a sickly glowing greenish tint, you know a tornado is somewhere near.
I’ve had someone attempt to explain this to me before, just why those vile green clouds. Had something to do with a combination of factors leading to the clouds “reflecting” vegetation on the ground. If that’s true, west Texas tornadoes wouldn’t have that association or at least not so pronounced. I’ve seen it several times before, it’s a very real thing. The whys and hows are a little more vague.