How can you tell when Boxer dogs are mad?

Recently I was watching some videos on youtube about boxer dogs playing, and there was a comment that said "look at how mad your dog is", there was also a response that said, "no my dog was playing, you will know when a boxer is mad." So how can you tell if they are mad?

When they have their teeth around your neck.......

Almost all dogs show the same signs when they are angry, these include growling, barking, and raising their hackles to bear their teeth.

When they growl and bare their teeth

my boxer puppy makes loud snarling noises when she is playing and sounds very aggressive, but if i was to put my hand near her mouth, she would never hurt me, at most she would gently 'mouth' my hand, which is something she has learned growing up ... boxers , like all dogs are capable of being aggressive .. they show this by snarling, staring at whatever is annoying them, and low growling noises .... this is my 3rd boxer and i have never seen extreme aggression within them, they are generally loyal, play full and loving dogs ... and very crazy!!