Just as is the case with dogs and other animals, there is a lot you can learn about future temperments when a rat is very young. You do this by observing play! If you watch a group of rats at play, you will note that there are certain individuals that are initiating more of the play and are the ones that generally attack and pin another cagemate. But, as these rats grow into adulthood, it is actually the more passive youngster that becomes the dominant rat. This is believed to be due to the defensive strategies learned by the rat that was pinned so often as a younger rat. Sexual maturity seems to be the turning point for both the dominant and the passive rat, essentially switching roles into adulthood. This play dominance is NOT an indicator of aggressiveness, though, as the play fighting is not associated with serious fights. There is a big difference in mannerisms with serious fights. Playful attacks are associated with targeting the nape of the neck, while more serious attacks target the rump and flank. Rats play as adults too, often in boxing or sidling positions, but again with target to the nape of the neck. Play fighting is juvenile in nature even as adults, and will often include one rat exposing it's belly or chasing and fleeing behavior. As adults, these behaviors continue, but may be replaced by boxing, kicking, and forequarter pivoting. Rats tend to peep and squeak at one another in play fighting too. With serious fights, hissing and loud, long squeaks of warning are more common. Typically, defensive moves include open-mouth display of teeth, hissing, nosing off, tail writhing, piloerection of fur (poofing), boxing, sidling, kicking, nipping at the face, and belly up postures. Interestingly, though, the belly up is not a submissive posture in rats. It is defensive in the way that it covers the areas of the rump and nape at which attacks are directed, whether play or not. Offensive moves include chasing, sidling, kicking, and biting.
And what about play dominance heirarchy with larger groups of rats? In this case, there is an alpha or dominant single rat and those around that rat take on a couple different roles as subordinates. Playful contact between these individuals is an important aspect of friendship and bonding. But, there are two main strategies that subordinates take. One is for the subordinate to maintain close contact with the dominant rat (beta subordinate) or they avoid him (omega subordinate). Those that avoid the dominant rat tend to be more aggressively treated and are more likely to leave the colony if possible, but this rat is also in a stronger position to become the dominant rat should the current one leave or pass. The beta is more submissive and more tolerated by the dominant rat, so has more access to resources as well, however this rat has less of a chance of becoming a dominant figure and will likely remain a subordinate to another dominant rat even if the current one leaves or passes. Temperment is influenced by genetics that reflect on a rat's boldness, and therefore, it's choice of dominance/subordinance.
Seperate from this, but still very much a part of the picture, is allogrooming, or the grooming of another rat. Nibbling during grooming is associated with play and will often illicit peeps of protest from the groomed rat. However, grooming around the face is also a social behavior, maintaining bonds between rats.  This differs from self grooming, which involves scratching with the hind feet, grooming the face, flanks, and tail, and nibbling. Excessive nibbling can turn into barbering, where fur is actually nibbled off. Bald spots are most typical on the head, face, neck and shoulders and barbering can be done by one rat to another as well. Self barbering can also occur on the forearms and chest and is usually indicitive of either mites or a high fat diet. Calorie deficiencies, hormone imbalances, and stressful situations can also cause the barbering, though.

References:
www.spflrc.org/user/rats/WildRats.htm
www.spflrc.org/user/rats/RatPlay.htm
www.spflrc.org/user/rats/norway_rat_ethogram.htm
www.spflrc.org/user/rats/Glossary.htm