I would find a mechanic you trust or independent shop. Get them to go over the car, explain some specific concerns.
DD missed a seat bolt recall. The front right side seat bolt was MISSING. They told me they would rush one to me, it came 3 days later....and when I found the holes in the carpet he said they would have found that in the inspection and they couldn't be that bad. Then he tried to bring up that my trade in car (which they had on the lot) died on a test drive, it was a dead battery, he was accusing me like it was my fault!!
There where holes cut in the carpet under the floor mats for a cage. The exhaust they installed was hitting the heat shield from the moment I drove it off the lot. The pill in the turbo tube was m.i.a. so the car was not boosting the full amount. A power steering hose clamp was loose. There was a blot missing on the side of the dash(when you open the door). The salesman was very nice and helpfu but flat out lied to me. Tried to show me some paper in his hand that he said were the service records from DD that showed the car was in for it's regular servoce and there were notes saying it wasn't modified to which I found out that was a lie.
I would have bought the car regardless, for a better price obviously, as it is tough to find a unmodded or molested WRX or STI. Don't fool yourself into thinking otherwise.
Those are the kind of things to look for.