I can’t say for certain if they’ve gone through the portal, and I doubt it would be possible to know for certain unless you go through yourself. Even psychics don’t quite have the reach to tap into minds on different planes of existence. (Don’t ask me how I know this.)
However, I wouldn’t recommend going through yourself. That’s a dangerous move; after all, who knows what could be on the other side? It could indeed be a mirror of our own universe, or it could be a place full of dangerous and powerful pokémon. Dimensional travel isn’t always straightforward either, and you don’t know for certain whether or not the portal is stable enough to support a return trip.
I would recommend contacting the rest of your team and formulating a plan together. Best of luck, anonymous.