It's natural for a turtle person to come to this conclusion, as seeing turtles in the wild is rare for most folks. If you live in the range for this species you should not consider this a possibility.
1) the probability of finding a turtle that was a former captive is infinitesmally small. You might as well win the lottery while getting hit by falling airplane parts.
2) it would be irrelevant if it were once a pet if it is inside it's natural range. turtles do well when released (as evidenced by the spread of the RES across continents from pet releases)
3) wild turtles will often not act as you expect. I have hand fed wild turtles from my own fingers and had some of my own captives refuse to come out of their shell for years (some even of my own captive bred). I've only found two turtles in the last two years in the wild that would not extend themselves for me to get a good look within seconds of being picked up.
4) most of the turtles I find have few if any markings that look like injuries.
5) bumpy shells can be caused by wild nutritional factors just as easily as captive ones. It's less common to see partially because the animal is less likely to survive such conditions in the wild or in captivity.