The slot in the hitch where the trailer king pin latches is usually placed about 6 inches ahead of the rear axle and on the truck's centerline. There is somesmall latitude in fore/aft placement. Further forward helps shift weight to the front axle and makes the trailer more easily maneuverable when backing but also makes the trailer "lag" (cut the corner) a bit more when turning in the forward direction. Closer to the truck's rear axle makes the that axle carry more weight and requires that you turn the truck steering wheel more radically when backing.
In any case, measure the distance between the hitch location and the back of the cab and then try to figure out if the trailer will hit the cab in a turn. Essentially you want 1/2 the width of the trailer plus an inch, but a couplel factors complicate how this is determined. The king pin may be forward from the front of the trailer (an extended pin box) or be behind the front surface or the trailer may have rounded corners. All these change the geomtery of where the front corners go in a turn. However, it is not necessary to allow clearance for a full 90 degree turn but be sure to remember when backing up so that you do not accidentally damage cab and trailer. (There is almost no situation going forward where you would ever get at a 90 degree angle.)
