What color is the fruit inside? Cherry would be dark as the skin, I think?
I agree it is likely a plum if it has a pit.
As for the vegetable plant, I don't think anyone can tell until it actually produces something. Cucumbers look like squash and vice versa at that stage. (And maybe someone who knows more than I do will speak up and give us a better clue!
) At any rate I have never been able to identify such volunteers at that stage.