When is it summer?
The first really hazy hot day that builds to local thunder showers, or the sound of cicadas would both qualify, but those aren't early signs. The first Queen Anne's lace (daucus carota) or the first blooms of the brilliant orange Asiatic lilies in our gardens are earlier signs. The sweet evening songs of wood thrushes are a bit too early, arriving actually in late spring, but I think of them as a summer sound.
For me, the light is the most direct indicator. I am sensitive to the quality and intensity of the sunlight, and by late May it's no longer a spring sun.
The second growth on the trees is another message I receive without conscious notice. In the spring there is an exuberant first growth of leaves on the trees and shrubs. This leads to the unique spring blend of different colors and textures as the leaves are in their infancy. After those leaves reach full color and size, though, the trees pause for a few weeks, then put on another seasonal growth spurt, reaching further into the light and filling all the gaps in their foliage. This creates an overstuffed, shaggier appearance overall, and I tune in to this.
And of course there are the signs at the grocery store. The arrival of fresh local strawberries is an early warning sign. Watermelons are an even stronger indication. Then it's time for cooking burgers on the grill (actually, that's an all year thing in this house) and making potato salad, cole slaw, and corn on the cob.