For the oauth2.txt admin credentials, you'd want to look at the client_id field in that file and make sure it matches the client_id in the client_secrets.json file. If it does then the project_id in client_secrets.json is the project being used by the oauth2.txt admin credentials.
For the oauth2service.json service account credentials, the project_id for the service account should be directly in this file.
Generally on a given GAM install both of these should be the same project ID but if you've redone GAM a few times it's possible to have them split between two projects (something you'd probably want to clean up also).