An orthopantomogram (OPT or OPG) is a useful investigation to assess the cause of the dental abscess, and to help exclude conditions which may clinically appear similar (a sebaceous cyst on the face can mimic the appearance of a dental abscess). The OPT will also help identify the causative tooth, and should emergency treatment be required, this x-ray can be vital to ensure the correct tooth is removed.
If patients are unable to tolerate OPT radiographs (patients may be unable to stand still due to neurological or motor diseases such as dementia etc), then a lateral oblique mandible of the area can be beneficial (if it is a mandibular abscess). However this should only be used as a second line view, as it does not include the full detail provided by an OPT.
The image below shows an example OPT, with arrows showing carious lesions, and the periapical (around the tip of the tooth root) radiolucency classic of dental abscess. It may be worthwhile discussing these images with your local maxillofacial unit if there are concerns, as sometimes the changes can be more subtle.
Reference: Wetherell et al. Management of acute dental pain: a practical approach for primary health care providers. Aust Prescr 2001;24:144–8.