There are a couple ways to know which cards are included in each expansion. All cards have a watermark behind the description which indicates the expansion they belong to, but it is a bit difficult to recognize them that way.

A better option is to filter cards in the collection by expansion. Click on the 'search' field, and it will bring up a list of options to filter by, including by expansion.

Lastly, you can also check the card list on the official website, where they are organized by expansion:

https://www.horusheresylegions.com/legions-card-list/