In discussions with other members of the Navy, I have reached the conclusion that there are two catagories of Naval personnel.
Sailors: These are the true blue, all-American, proud of their uniform, follow orders, squared-away, sailors. These guys follow the rules (to a greater or lesser degree) and tend to stay out of trouble.
Squid: These are the sailors of legend. They are the ones who never shy away from (if not start) a barfight. They drink a lot, they solicit the services of prostitutes. Squids have all at one point or another done time in either a foreign jail (waiting to be picked up by their respective ships) or in a brig, most likely serving a punishment of 3 days bread and water. They swear fluently and colorfully.
One method of differentiating the two types is to look at their Dixie Cups. (Dixie cup is the colloquial term for the white, bowl-shaped hats worn with dress uniforms.) Sailors' dixie cups are perfectly round, and generally blindingly white. Squids' dixie cups have 'wings' (The hat is pulled and stretched off the left and right side, forming it into a vaguely paper boat sort of shape, and worn so that it presents a wider profile when viewed from the front) and tend to be a dingy yellowish color from smoke, beer, and occasionally bodily fluids.
A second method of detection is inside the cuff of their dress blue uniform. In all but a few cases, there will be some sort of patch sewn into the inner cuff of squids' dress blues. The design will vary depending on the place where it was bought, but it is generally a Chinese dragon or something equally testosterone laden. Sailors don't have them because they're against the rules.
The third and final method is by checking the neckerchief which is the black tie that is worn with dress uniforms. A sailor's will be neatly pressed and expertly rolled. A squid's will look generally similar, but once removed, they will have a roll of quarters or some other similar blunt heavy object sewn into the back of the neckerchief (the portion which lays across the back of the neck, under the 'cape' of the dress uniforms). The reason for this is because, when swung bolo-like, it makes a great weapon in the event of a "Friendly Disagreement" at the local drinking establishment.
Thus concludes the Field Spotters Guide to Naval Personnel.