Fourth, check to see what you know or can find out about the objects that appear in the photographs. Do you know or can you measure some 3D locations in the scene as it exists now? If you cannot get 3D locations of points by surveying or by knowing something about one of the objects in the scene then you will not be able to use control based Inverse Camera.
The known information can be in the form of XYZ Control Points or in the form of Axes Constraints. See the Single Photo tutorial for an example of using Axes Constraints to solve for focal length.
If you are using Control Points to do Inverse Camera processing, you must meet some minimum requirements. If you are solving just focal length in Inverse Camera (most common) then each photograph, that has Inverse Camera focal length turned on, must have four or more Control Points with a good spread. If you have asked for principal point solution also you will need an additional two Control Points and for format size an additional one Control Point. So if you have all three Inverse Camera parameters turned on for solution you will need at least seven Control Points on that photo.
Good example: You have two photographs of a car in which the front has been crushed in. The photographs show large parts of the undamaged vehicle, and you have easy access to an undamaged exemplar vehicle of the same make and year. You can measure the exemplar vehicle with PhotoModeler and use the output DXF as the input source of control for Inverse Camera on the unknown photographs. You can obtain 15 or more control points to mark on most of the unknown photos.
Fair example: You can see an ambulance in a couple of photographs, and you can get the dimensions of the rear box, but the photographs are taken in two-point perspective (the vertical edges of the box appear to be straight up and down in the photos). You have a source of control, but the control is not very strong. If you need to recover focal length, principal point and format size you may have trouble.
Bad example: You have some photographs of an accident scene of a very rare automobile. The automobile has been crushed. The road scene it was photographed in has changed (the road has been repaved; the surrounding buildings have been torn down). You have no source of known points in the photographs! This project probably cannot be done.