When you have multiple publishing destinations configured, you can easily switch server by clicking the drop-down menu beside the Publish button and selecting a different destination.

By default, RapidWeaver gives each destination a generic name, but to make it easier to identify them you can change the name by going to the Publishing item in the Page List. Then, simply click on the name at the top of each destination and start typing. The name of the currently-selected destination will show under the toolbar’s Publish item.