How do I tell if I have a flat spot?

BFG KM3s 285/70r18: I have this annoying bounce in the rear like I have an out of balance tire. I’ve gone back over my lift & had Discount tire rebalance and rotate. I’ve locked the brakes up 2-3 times and although ABS kicked in I heard a skreetch and enjoyed the look in the rear view ahead of me of someone shitting themselves for cutting me off. Maybe I have a flat spot? How can I figure it out?
 

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