A "Chat" origin for your cases will allow you to easily filter cases which were created from a chat activity with a customer.
Step 1: Create a new Origin label for chat cases.
To create the origin first to to Settings | Customizations:
Go to Customize the System:
Under Option Sets double click the Case Origin entry:
Now click the + under Options, this will open an area for you to create a new Origin type e.g. :
Step 2: Test the new Case label.
Now that the origin has been created you can test this by starting a new chat. On your Agent Widget click to Open chat activity in Dynamics 365:
Use the To Case button to create a case from the chat activity:
Fill in the details (i.e. select the customer, subject type etc)
Set the origin of the case to your label (I used Chat), you will also see that the chat activity is now linked with this case.
Don't forget to save (bottom right corner)
We can now go to Service | Cases to list our open cases. 
In the list of Cases you will see your newly created case. The Origin column should display your new label.