Your sketch (and your previous sketch) both seem more than adequate to me. Someone else has proposed a controlled test of a claimant (such a test has been on offer for decades, with a sizeable reward; nobody has ever collected the reward). Here's another test one:
A passport database and facial recognition software. That would have everything you really need: standardized photo methodology, associated birth dates, and a programmatic way to gather the data.
Great idea. I'd hire you.
Then throw some statisticians at the data set--or even just statistical analysis AIs. You could mask the data so the analyzers didn't even know what they were looking at. Throw enough computing power at the data for long enough, and I'm sure some kind of pattern would emerge. My bet is that you'd find a much stronger correlation between age and facial features than anything else, and any birth date correlation would be lost in the noise.
Can you explain what you're looking for more clearly, beyond the tests you and others have already described?
Mainly to know whether or not the guy was on the level because of how impressive his success rate appeared to be
No useful correlation between birth sign and measurable characteristics in humans has ever been demonstrated. It is doubtful than anyone will ever bother to make a scientific test for your specific scenario, nor is such a test necessary to resolve the conundrum posed by your dinner guest.
That conundrum is reasonably resolved by what we already know about physics, biology, astrology, stage magic, and charlatanism.
Detractors will not be impressed, but I saw no way he could have gleaned their signs at the table before he did his thing. He missed 2 or 3 out of 10 or 12 people. After explaining his skill, he looked around from face to face and named their sign. In the circumstances and time he had, I saw no way he could glean the info. He walked into a room of strangers and sat down. He would have had to steer conversation to birthdates for all or most persons to get those results. That didnt happen.
So I came up with the specific study method sketched above. One member implied it has not been done. Meanwhile I'll start with Shawn Carlson.