Go outside at night and listen for their calls. You may also find them in developed areas hanging around artificial lights at night, they learn to stake them out for the insects that are attracted to the light. I'm not sure what it is like now, but I remember taking a trip to WI for a family reunion in the early 90s and the ground was covered in newly metamorphosed gray tree frogs and American toads, it was almost impossible to avoid stepping on them.