How do you know if our dog is playing agressvely with your or playfully?

Like when i play tug of war with my puppy I cant tell if hes being agressive. His tail is up but not wagging, and he seems like hes trying to i somethng against me. and one time he was growling.How do you know if our dog is playing agressvely with your or playfully?
Its most likely playing.

But try when your playing with him to always win.

This will show him thats you always win and your the dominant one.(the leader of the pack[or house])

for example if your play tug o war always make sure he lets go before you, teach him to release. or leave it. (:



Also I suggest that every dog owner watches this video :D

http://www.youtube.com/watch?v=GOW0IKO_z鈥?/a>How do you know if our dog is playing agressvely with your or playfully?
Ignorant kids ruin dogs,because they don't know how to handle/train/play.
when his head is not wagglling side to side, he plays agressively.
It would be hard to tell if you were just playing tug of war...dogs get excited and carried away just like people. If the growling is directed towards you then you need to address that immediately...be stern and forceful with him right away...like someone else said in here...dogs need to know you are the one in charge