My question is probably very stupid, but .... I have installed 2 servers with SharePoint 2010, i have created one web application. My question is: how to know if my server is a WFE or an application server. Thanks
Since all servers in a SharePoint farm have the capacity to be WFEs, the most obvious way to determine which one is the WFE is based on which one is accessed by your main site URL. If your url of 'somesite.company.com' goes to server1, then than is your WFE. A tracert or a ping can be useful in tracking this if you are unsure.
There are a lot of other distinctions, like what services run where and so forth, but this is the easiest method.
The role of the server decides what type of server it is: http://technet.microsoft.com/en-us/library/cc261752(v=office.14).aspx
In other words: It depends on what services you start on the server.