Does a dentist know through a regular digital dental x-ray that a person needs a sinus lift or is there a more reliable image method?

Type of scans: In many cases, all that is needed is a 2d conventional x-ray to determine that a sinus lift is necessary for the placement of a dental implant. There are more accurate and revealing image methods such as a cat scan or cbct (cone beam computerized tomography) 3d image.

Yes...but: If a regular digital x ray shows the need for a sinus lift for implant placement that is not going to change with a 3d image.The height of the bone will be the same and either there is sufficient height or there is not. A 3d image helps in angulation of the placement of the implant and determining the thickness of bone. These are not usually as much of a concern for upper back tooth area.