I picked off a mole and I'm afraid it was melanoma. if I do a skin excision at a plastic surgeon off the area from the place where the mole was and sent it for biopsy. Could they see if there was an cancerous mole there.

They can see

even if the mole has been scratched away?

?

Yes even than.
But scrathing away a mole, doesn't change it into a cancerous thing.
Moles/Birthmarks are a common phenomenon. And there is actually no reason to assume that they are malignant, unless they suddenly change shape or cause other complaints.

And if you were afraid it was melanoma, you should have gone to the doctor before. But apparently the fear only arose because of the scratching away.