As the fruit gets ripe, birds will eat them also wasps and butterflies will feed off any rotting fruit that may fall to the ground. Then you will know it is safe to eat.

I don't know of anything that has fruit that looks like a greengage that isn't safe to eat, though if it's a darker plum variety that isn't ripe yet, it wouldn't taste nice!

Thanks both - that's helpful and reassuring. The tree is laden with fruit!

As similar shrub to the greengage is Chaenomeles, Japonica or Japanese Quince. It has spikey stems with green plum-like fruits. I don't think people would eat as a desert fruit but I believe they make a jelly out of it.

Look for the typical stone fruit groove on one side of the fruit, if it is a greengage. If the fruit don't have grooves, get better identification before eating!!

Thanks - definitely not spiky, but has the groove, so looking good for greengages!