There is no apex locator at the clinic where I work, so I am trying to determine the working length using digital radiographs. Do you have any advice?

Working length can be estimated on a digital radiograph (RVG), but my first recommendation would be to obtain an electronic apex locator for the clinic. The recommended approach is to use electronic measurement together with radiographic assessment.

An important limitation of radiographs is that the root tip seen on the image does not always coincide with the point where the canal opens onto the root surface. Working 1 mm short of the radiographic apex therefore does not guarantee the correct working length in every tooth. A file tip that appears short of the root tip on the image may actually extend beyond the apical foramen.

Before obturation, you can try the paper point test. Mark the paper points at the length measured radiographically and insert them to that depth. After a few points, one should come out dry. If it does not, the point may be extending beyond the canal. This can give you an indication of canal length, but the method has limitations. It requires complete removal of the pulp, no exudate from the periapical tissues, and periapical tissues that are not excessively dry.

When taking radiographs, try to use the paralleling technique whenever possible.

If roots or files are superimposed, an image taken at a different horizontal angle can help.