This page of the Call of Duty Cold War guide provides information about evidence. You will find out where you can check the materials collected so far and how to identify the evidence to be collected during the campaign.
Where can I check the collected materials?
You can check the collected evidence on the Evidence Board in the main base between missions.
When you click on any mission on the evidence board, you will be able to view the collected materials (as in the photo below).
There's not much point in browsing the evidence for the main missions, unless you want to play immersively and learn more about the universe. It only makes sense to look at the collected materials during side missions: Operation Red Circus and Operation Chaos. During these mission you will have to connect the dots using the evidence.
For more information about the evidence related to these missions, see the Evidence List.
Evidence - where can I find it?
The main campaign does not require any search at all. However, evidence gains more importance in the case of side missions. You can obtain evidence in two ways - by interrogating suspects or by finding maps, diaries, watches and recordings in campaign missions.
Look around the locations, search for collectibles located on desks and walls.
Remember to have a solid look-around in the following missions: Brick in the Wall; Redlight, Greenlight; Echoes of a Cold War and Desperate Measures.
To learn more about evidence, its locations and significance in the game, go to the Evidence List page.