Hager Companies offers 4 series of door closers. The 4 offerings are the 5100, 5200,5300 and 5400 series.
The first way is to look and see if there is a sticker on the body of the closer.
Here is an example of the 5100 series closer body with label
The other way is to identify the physical look of each series body.
Examples of each series:
5100 Series
5200 Series
5300 Series
5400 Series