The baby changing tables in public restrooms are usually kept pretty clean however I would recommend wetting a paper towel down with some soap and water and wiping the baby changing table down just to be safe.
Or use some baby wipes to wipe the changing table down before placing your baby on the changing table.
You should also use some changing pads on the changing table and usually the changing table should have some disposable changing pads that you lay down on the changing table to change your baby.
If not place some paper towels down onto the changing table before putting your baby down on the changing table as it keeps the changing table from getting dirty from your babies diaper contents as well.