Before (© Michael Stappen)
After (© Gerrit Griem)
Airliners with satellite (good) wifi have a prominent circular bulge on top. For example, this plane was retrofitted with satellite:
Before (© Michael Stappen)
After (© Gerrit Griem)
So, you can find whether a flight will have satellite wifi by viewing the exterior of the plane at the airport, or by researching the expected tail number (N395DN in the example above) and then using that to look up pictures of the plane.