I’d go with the arms-crossed-over-chest salute the aliens used in Plan 9 from Outer Space. (No one else is using it and Dopers can reasonably be expected to know it.)
I went to Las Vegas for a Dopefest (Hi Kurilla! Hi DMark!) and everyone I met was a Doper. :eek:
What are the odds of that? 
Numerous body piercings with this will surely elicit comments from fellow Dopers whom one encounters.
Where’s the straw?
1st person: Humans have been to the marianas trench…
2nd person: But only once…
1st person: For 20 minutes…
4th person: Cite?
The 3rd person is just lurking?
If you search well on the internet- at one particular, very popular, free WAP site there is a Straight Dope wallpaper available for your cellphone. If you flash the the red, blue, and yellow colors of the Dope from your phone at the suspected doper and their eyes glaze over in a hynogogic state, chances are, they are a Doper.
The 3rd person is Opal.
The third person is AlWAYS Opal.
No, they have dial up and haven’t posted yet.
Do the sign of the devil. Now, bend your wrist and point it down.
That’s Pi.
π
Shortened a bit …
Doper #1 Hi! How’s it going?
Doper #2 Great! Did you bring pie?
…remember to bring Pi.
Put the back of your right hand against your right temple and then bend your index finger till it’s parallel to your eye; this indicates “sight” which is a homonym for “cite?” which stands for pool (but will ultimately get you pitted by somebody complaining it’s unfair to Dopers without right hands).
Doper in the back of 1st Bentley: “Pardon me sir…Would you happen to have any ‘Opal’s 1920’s Style Trench Pie’?”
Doper in the back of 2nd Bentley: “But of course!”