Glass: When you're empty the server just asks if you want another, not before.
Ceramic Mug (without a pewter lip/top): Lay it over on its side for a refill.
Ceramic Mug (with a pewter lip/top): Closed means you're OK. Open means a refill. If you look like you don't know what you're doing and leave it open they will check and correct your "gasthaus etiquette".
Other than that they can read the "glasses" from across the room and don't bother you like American servers do.
In Germany you also get a new glass not just a refilled old glass. The glasses have markings on the side (near the top) that indicate how much beer is supposed to be in the glass - it's a law. If you get one that is below the line you can send it back to be filled. If you've been in the gasthaus most of the night and/or are a regular and are close to leaving but don't want a full beer then you can ask for a "schnitt" (which means to cut or slice) which is a half beer or a fast pour. Sometimes they won't even charge you for it.
You also don't get/order another unless the other one is empty first.