If you have two puppies, they are playing. Play can be very noisy and look quite rough, but both dogs will walk away unscathed and it's important for them to learn bite inhibition and the like from playing.
But if you're interested, if your dogs have had or are having a fight, there will be blood. Fighting is generally a last resort for most dogs, though I have seen exceptions in terriers of any type (including bully breeds), cattle dogs and mixed breeds with these breeds in them.
It can be confusing because they can perform physical displays to convey a message - for example, dominance displays. They will try to be as threatening as possible and the noises they make will be loud and a different sort of growling from when they play. That said, they acknowledge that they are both in the same pack and there is no real desire to hurt the other, so again both walk away unscathed. These displays can turn into fights if neither backs down and particularly, if the human is not the pack leader. Here is a great example of a dominance display Husky and Lab Territorial fight - YouTube This is the most common type of 'fighting' that I see. Note that neither wants to hurt the other, they want to keep the other dog in the pack, they just want submission.
On occasion however, 2 dogs will take offense to each other, whether it's over food, resources (including the humans), sex (access to mates) and/or territory. When they are fighting, they will no longer be trying to get on top of the other dog to dominate them, they will now instead be going mainly for 2 areas - the underside including the neck and/or the legs. There will be biting, screaming, shaking and lots of blood. Depending again on the circumstances (and breed does play a role), they may stop once the other dog goes into complete submission or they may have decided that the other dog has to go in which case they will continue to try and kill it. I have seen that particularly with bitches in the same household, and virtually in every fight that involved a terrier of some sort. Here is a rather disturbing video, but it shows the sorts of sounds and the sorts of bites you're likely to see if one dog wants to really hurt another. http://www.youtube.com/watch?v=7wHrc0x_syo