In the past we used to just continue ti increment list IDs per contact. So if you have lists 1,2,3,4,5 delete 4, then add a new list, the new list would be ID 6.
However, this has been changed and we now assign a fully unique ID to every new list when it is created. Because of this, the best way to get a list ID is to make a GET request to the lists endpoint of the API so that you can get the ID there and use it in your code. If you simply need the list ID once and do not need to fetch it manually, you can make use of the I/O docs page that is now available through Mashery for testing API endpoints. You can find that here: https://constantcontact.mashery.com/io-docs
This page will allow you to take an API key and generate an Access Token for use with the API. Once you have the access token, you can run simple requests directly against the API from this page and view the results.