The PH of a solution can be measured using the H+ ions concentration present in the solution. PH= -log(H+) H+ is the hydrogen ion concentration. or by knowing the OH- ion concentration. PH= 14- (-log(OH-)-log(OH-) is the pOH of the solution.
If you see a brown patch under the stem it's going to be a little gross inside. If you see a nice yellow-green patch you have a perfectly ripe avocado. Just make sure nobody at the grocery store notices you flicking the stems off a ton of avocados.
On the other hand, green oranges are likely immature, although some varieties, like satsumas, can be ripe yet still show a little green. But while most ripe oranges will be a deep rich color, not all fruit that is a deep, ripe color will be ripe.