Trekkies are generally older, usually have at least one "family" photo which includes some type of VW automobile, and don't mind being called "Trekkers."

Trekkers usually love a good argument, have never been in a "sit-in" ever though they think it would be cool, and hate being called "Trekkies."

Trekkies enjoy putting together Star Trek models of phasers and starships.

Trekkers have the latest Omnipedia on CD and get irritated when they can't find a starship class in 3-D rendering.

Trekkies know that the best defense is a strong offense.

Trekkers know that the best defense is a Janeway Phaser Rifle 2000.

Trekkies bore you with endless stories about how their first con was a real con.

Trekkers bore you with recitations of endless continuity errors, academic-like articles about the application of the Prime Directive in the Real World, and multiple theories about childhood traumas of Star Trek characters.

Trekkies know that space is the "final frontier."

Trekkers know that space is divided up into a series of quadrants, but they're not quite sure how that figures into navigation charts.

Trekkies have a pair of Spock ears, or have at least once in their lives pulled at the tops of their own ears and tried that eyebrow trick.

Trekkers...hmmm, sorry. No difference here.

Finally, when confronted with someone who's really hard to categorize,
you can just say, "What do you think about Star Trek?"
If the response is: "Which one?" -- Trekker
"Love it!" -- Trekkie
Tips:
Writing me nasty emails about any of the above will only get you the comment, "I'm kidding," or a visit from my Janeway Phaser Rifle 2000.