You can tell how easily a rear wheel spins when climbing a familiar, steep hill on a wet day.
You can also tell how easily the rear wheel locks up when braking on wet roads, although this test has to be repeated deliberately or accidentally many times to smooth out the noise and provide meaningful information.
As you say, it’s hard to tell anything by cornering on slicks until it’s too late, even though it is in wet corners where the differences matter the most.
Of tyres I’ve used, the Michelin Pro4 Grip has had the best wet grip (very different from a Schwalbe One), but that is a draggy tyre.