Even if you have a guide as to “This variety of apples ripen in first week September”, your particular soil, sun exposure, micro-climate and other factors can throw that date off. Not only that, some varieties appear to be “indeterminate” in the whole tree not ripening at once (I’m looking at you, Honey Crisp!).
The three tests are 1) are any already falling from the tree, 2) do they pick easily (if you have grown apples, you already know this, but if they come off the tree, not by pulling or twisting (the “roll axis” of the stem) but by tipping up to bend the apple off its stem (the “pitch axis” of the stem).3) when you cut one open, are the seeds still white or are they starting to brown indicating ripeness.
Number 4) is taste one. Tartness, even extreme tartness is not a reason to say they are not ready if it is a tart apple. A green apple (term-of-art meaning not ripe, even if they are blushed as in your photos) will have a kind of woody-starchy-off taste. A neighbor with a commercial orchard sells large (he must blossom prune) green apples at his fruit stand during pick-your-own tart cherry season in Door County (Northeast Wisconsin, lake-moderated micro-climate between Green Bay and Lake Michigan), which is about end of 3rd week in July.
I ate one and besides being super crisp and super tart, it had this kind of starchy taste to it. A year later he explained to another customer “the variety is Lodi, and I pick them this early because some people like a crisp tart apple.” I picked some of my first crop from a Lodi one week later, and they were still crisp and tart but they didn’t have that “green” taste to them
:I would say it is more of a “forward slip” though, really.