I don’t know of a way to produce a serverless.yml with variables replaced but if you look in the .serverless folder you should see the two CloudFormation files that will show the values being sent to AWS.
Just to add to what @buggy said - the command you’re looking for is serverless package, which will create the CloudFormation template with your variables populated. It’s not in the format of serverless.yml, but will let you know if your config is good.