Installing a slab exterior door
The normal way of checking door jambs is with a sting line fasten the end to the bottom of one of the door jambs and run it to the top of the opposite jamb across the head and then down to the bottom of the other jamb.
So what you now have made is a X with the string line,the stings should just sit upon each other.
When there is a gap between the lines or one pushes on the other then the jambs are not parallel with each other and by moving the line in and out you can see where the door jamb must be moved to bring it back to true.
Check the door by putting a sting line or straight edge across opposite corners there should be no gap in the middle turn the door over and check again it could be the door.Hope this was of some help. Billy