There are four signs which tell if Victoria Plums are ready to pick:
Twist the plum gently off the stem. The plum is ripe when it comes off easily. Plums usually leave their stalk on the tree.
The colour of the plum changes when they are ripe. The skin becomes reddy-pink on one side & yellowy on the other.
The plum on the left of the picture is not quite ripe. It is greener than the plum on the left which has turned more yellowy as it has ripened.
Cutting a plum open will reveal the stone & show if it is loose inside the plum. This a a photo of the yellowy plum above after it was cut open. The stone was quite loose and fell out
ripe plum with loose stone
If the stone is still firmly attached to the flesh and difficult to separate, the plum is not ripe. The photo below is of the green plum in the picture above. The stone would not come away from the flesh.
unripe plum cut
Gently squeezing the plum will tell if they are still hard. If they are softening a bit, then they are ripe.
Victoria plums do not all ripen at once, so they are not all ready to pick at the same time & the tree is usually picked several times.