If it has been tuned with SERT then the seller should have the little orange box to give to you. If it hasn't been tuned for a freer breathing exhaust system then it's going to run even leaner than it was programmed by the factory and possibly run hotter on you. I'm not sure how to check if it's been retuned with a SERT but I'm sure someone here on the site will have an answer. Does it pop on deceleration when you close the throttle rapidly? Does it pop on decel when you slowly close the throttle? What grade of gas have you been using, 93, 89, etc? Were you provided all the maintenance records when you acquired it? Even with the SERT, if it was not tuned with your existing exhaust pipes or just tuned to some generic map, you could still have a deceleration pop on occasion. If it's doing it regularly, I might suspect a too rich fuel mixture setting on your map.