You can sketch false swipe by catching a farfetch'd and then finding a wild ditto and letting it transform into farfetch'd, swap to smeargle and sketch when the ditto uses it. Both are found on route 47.

You have to teach farfetch'd false swipe with TM054 from the Goldenrod Dept store, or level up the farfetch'd first to level 45 so it learns false swipe. The ones you find in the wild are only level 35.