Travel expert Stephen Au writes that when you use Google Flights, you can select a leg, then make sure to click on the dropdown arrow button to see the full details of the flight.
In these full details, Google Flights includes the name of the aircraft model. Other resources like SeatGuru offer similar information so you can avoid the Boeing 737 Max.
You can also find a list of all the airlines that don't operate Max jets. Delta, Jet Blue, and Spirit Airlines don't operate the Max, while American, United, and Southwest do.