I agree with Chaplin on this one. If something is grossly wrong you or your mechanic may catch it but there's no guarantee that major or minor problems won't slip by you. Check for water in the
oil and
oil in the water. Listen for unusual sounds. Beyond that and perhaps a compression check your best hope is just to look the car over and see if it's been well maintained. If the car is clean and well maintained, chances are the motor has been well taken care of too. If the guy is slimey, he may try and pass off a 390 as a 428 but I doubt it. If he was trying to pull that, he'd tell you that it was a 427. Without tearing the car apart it's virtually impossible to tell the different FE's apart. Good luck.
Chris