The DICOM standard defines UIDs for describing the hierarchy. These are from top to bottom:
- Study UID - Identifier of the study or scanning session.
- Series UID - The same within a series acquired in one scan.
- Image UID - Should be unique for any image.
A DICOM image saved by a standard-conforming implementation should have all these IDs. If multiple images have the same SeriesUID, they are a volume (or time-series) as defined in the standard. Some software of course is not standard-conforming and you'll have to look at other things like timestamps and patient position, but it is usually best to start by following the standard.
For ordering the series after identifying it, GDCM (as malat suggested) or dcmtkdicom are pretty well-established libraries.