aggressive barking is usually accosiated with raised hackles, raised tail or raised ears...I don't know how, but I can always tell whether a dog is nice or not lol ...sometimes I can tell they are mean even when they aren't barking..
Like this dog in a truck the other day, didn't move his body towards, me, just turned his head and looked really nervous and kept licking his lips like he was aggitated...yikes, you wouldn't see me going over there! lol