Valmikiramayana.net says:
By the way, where is the scene of Rama eating fruits and eatables bitten and tasted by Shabari? Has this old woman not offered the fruits to the guests, or just said that they are available? We listen many stories and see many pictures telling that she gave many fruits after biting them with her own teeth to test the taste. All that is not in Valmiki Ramayana. This amplification is from Padma Purana. phalaani ca supakvaani muulaani madhuraaNi ca | svayam aasaadya maadhuryam pariikShya paribhakShya ca | pashcaat nivedayaamaasa raaghaabhyaam dhR^iDhvrataa |
However this isn't found in padma purana(a verse number would be helpful). Do any other scripture mention about this?