Yep, probing is the best way. I can honestly say 'feels like butter' is something I've never experienced, and I suspect one that tender might fall apart coming off the pit.... So, the description I use is "as tender as a baked potato that is almost done". You will feel a little resistance going in and coming back out too. I start probing around 190°., and use a fat probe like an ice pick or a small diamether wooden chopstick. Many of mine are over 205° before they are right. I still overcook one on occasion. Excuse the white balance on this indoor shot on a red cutting board... but you can see how thick I had to slice this one to keep it together
KCBS - CBJ
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Barbecue is not rocket surgery