Basically, Gojo can see if you do have a domain
In his fight against Jogo, Gojo was continuesly indicating to be aware of Jogo having a domain expansion after only having seen him for the first time. He explains Yuji that they'll go learn something about domain expansion. It's not just about flexing his own domain in front of Yuji because he could just have flexed his domain on another day priorly.
Gojo was even waiting for (didn't just pop his own) and provocating Jogo so that he pops his domain first. Gojo likely wanted a 'I'm in an enemy's domain' situation and later even explained Yuji all the ways how to deal with a domain and some properties of a clash.
"Oh, this is perfect." He wasn't stunned or surprised at all when Jogo popped his domain but smiled slightly.
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I don't know how much this matters for JJK scaling but wouldn't this be a Geto Upscale theoretically? Gojo didn't see the difference between Kenjaku or Geto with his Six Eyes in Shibuya. And by what they told him it couldn't have been a double. So Geto probably had had some kind of domain in JJK 0.