How to check if my SharePoint SQL server is a SharePoint web front end?
The service which makes a SharePoint server web front end is Microsoft SharePoint Foundation Web Application service. If it is started in a server, it becomes web front end.
The services will be available in all servers. However, it is the status (started/stopped) which determines the role. That can be checked by going into CA->Application Management -> Service Applications -> Manage services on Server.