It kind of started when I got this T-shirt as a present two years ago:
It is not just a slogan that is quickly filtered out under the heading 'generic ad-like content'. It invites checking where the error is. It is kind of a challenge - at least for suitably minded persons. Exactly that kind of person I'd like to get in touch with more. This T-shirt signals: "I'm a nerd and proud of it." And the positive feedback I got from this was part of the reason I chose to signal this more. Maybe you'd like to signal this too. Please remember the T-shirt alone will not do it. You still have to talk to people. For the introverted among us (me included) I recommend active listening.
The remainder of this post lists some slogans I have tried, some I will likely try shortly and other related resources.
Obviously I'm not the only one using this signalling approach. T-shirt dealers have lots of these in stock. Thus some I just ordered online. But the most effective shirt I 'designed' myself. It is a black shirt suitable for business purposes and such a shirt with a small slogal on it stands out. I chose
That which can be
destroyed by the
truth should be.
— P.C. Hodgell
which I'm delighted to also find recommended by EY here.
I also have this T-shirt about the Map-Territory correspondence:
"This T-shirt is black"
is true if
This T-shirt is black.
(I had this printed on a dark blue T-shirt thus adding a gradual truth aspect).
I plan to print more of my own design shortly. The next one will be
from one example.
At least, I do."
-- Vlad Taltos
You can find out more about this quote here. What I like most about this one is that it is a bit self-deprecating which if combined with otherwise high status signalling (stance and gaze) I take comes across as approachable.
More ideas I might or might not print:
"Two people acting
rationally  and with
 cannot agree to
-- R. J. Aumann
"When you have eliminated the
impossible whatever remains
however improbable must be the truth."
-- Sherlock Holmes
"truth resists simplicity"
-- John Green
"Those who do not move,
do not notice their chains.”
is such an ugly phrase.
I prefer world optimisation.
-- Harry Potter-Evans-Verres
So this is my personal approach to rationality signalling, but maybe you are inspired by it. You may use the comments to propose other such slogans, discuss these, question or laud the whole approach or propose other avenues for such signalling.