If your door appears to be warped because it doesn't fit tight to the door stop or weather stripping, it could be caused by an installation problem. The prehung unit could be out of plumb with the wall. Doors when installed properly need to be plumbed sideways (hinge to knob side) and inside to outside.
This basically means that the lockside frame and the hinge side frame are twisted in the rough opening. The door needs to follow the same plane as the hinge side. If the lock side doesn't line up perfectly with the hinge side, the door will appear to be extending into the room or not fit tightly to the door stop. This will give the appearance of a warped door.
You can check to see if the frame is twisted with a 6' level. Place the level on the length of the hinge side noting where the bubble is in the level. Now check the latch side noting the bubbles need to be in exactly the same location. If not you may need to adjust your frame. This can be caused by walls out of plane with each other (mis-aligned bottom wall plates),or just a poorly constructed wall.
If all looks good with the door jambs, the next step is to check for a warped door. The best way to check a door for warp is with the string test. You will need a medium to light gauge string.
1. Take the string and wrap or tape it to the top of the door.
2. Pull the string tight to the bottom of the door. If the door is warped it will show up as a gap between the string and the face of the door.
3. Check the door from corner to corner on both sides of the door. You will then surely know which direction the door is twisted.