A lot of studios will tweet out what they have at Pax about a week in advance, or put out a little news article if they're a larger company, such as microsoft or nintendo.
Aside from that you can do some clever guess work such as looking at the exhibitor list and then seeing what new games those companies are developing and/or have upcoming release dates.
One thing I would say though is that in past years, the console and hardware manufactures will have playable games that they themselves did not make, so you may luck out if you want to test a AAA title whose developer isn't there per se. For example, razer had Deep Rock galactic as a playable demo even though the company wasn't officially on the exhibitor list.
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Aside from that you can do some clever guess work such as looking at the exhibitor list and then seeing what new games those companies are developing and/or have upcoming release dates.
One thing I would say though is that in past years, the console and hardware manufactures will have playable games that they themselves did not make, so you may luck out if you want to test a AAA title whose developer isn't there per se. For example, razer had Deep Rock galactic as a playable demo even though the company wasn't officially on the exhibitor list.