There are various ways in which you can tell if a watermelon is ripe

August 27

See Late Blackberries

H. D. Thoreau, Journal, August 27, 1859

All our life is a persistent dreaming awake. See November 12, 1859 ("I do not know how to distinguish between our waking life and a dream. Are we not always living the life that we imagine we are?"); October 29, 1857 ("There are some things of which I cannot at once tell whether I have dreamed them or they are real"); August 8, 1852 ("When the play - it may be the tragedy of life - is over, the spectator goes his way. It was a kind of fiction, a work of the imagination only, so far as he was concerned.") May 24, 1851 ("I frequently awake with an atmosphere about me as if my unremembered dreams had been divine, as if my spirit had journeyed to its native place”).

I often see yarrow with a delicate pink tint. See July 5, 1856 ("Pink-colored yarrow.")

The children have done bringing huckleberries to sell for nearly a week. See July 13, 1852 ("It is impossible to say what day — almost what week — the huckleberries begin to be ripe, unless you are acquainted with, and daily visit, every huckleberry bush in the town"); August 4, 1852 (“Most huckleberries and blueberries and low blackberries are in their prime now.”); August 4, 1854 ("On this hill (Smith's) the bushes are black with huckleberries. ...Now in their prime. Some glossy black, some dull black, some blue; and patches of Vaccinium vacillans inter mixed."); August 4. 1856 ("This favorable moist weather has expanded some of the huckleberries to the size of bullets"); August 28, 1856 ("Huckleberries are about given up")

Perfectly fresh and large low blackberries, peculiarly sweet and soft, in the shade of the pines at Thrush Alley,-- so much sweeter, tenderer, and larger. See July 31, 1856 (“How thick the berries — low blackberries, Vaccinium vacillans, and huckleberries — on the side of Fair Haven Hill! ”) August 4, 1852 (“Most huckleberries and blueberries and low blackberries are in their prime now.”); August 19, 1856 ("What countless varieties of low blackberries! Here, in this open pine grove, I pluck some large fresh and very sweet ones when they are mostly gone without. So they are continued a little longer to us"); August 28, 1856 (“low blackberries done, high blackberries still to be had.”); 

Elder-berry clusters swell and become heavy and therefore droop, bending the bushes down, just in proportion as they ripen. See   August 22, 1852 ("The elder bushes are weighed down with fruit partially turned, and are still in bloom at the extremities of their twigs."); August 23, 1856  ("Elder-berries, now looking purple, are weighing down the bushes along fences by their abundance."); August 29, 1854 ("The cymes of elder-berries, black with fruit, are now conspicuous.")