If their tongue is sticking out slightly, it could be because of an overbite. An overbite is a condition where the top teeth are far too forward compared to the position of the lower teeth, thereby allowing a gap...for the tongue to stick out.
http://www.tucsoncitizen.com/index.p...5e1_dog_braces
IMO, when their tongue hangs out a little, that is a sign of an overbite. When their tongue hangs out a lot, that is just because they are happy or trying to cool down.