Sayyidah Aishah (may Allah be pleased with her), relating the affair of Khaybar, says that the Prophet (peace and blessings be upon him) used to send Abdullah bin Rawahah (may Allah be pleased with him) to the Jews, and he would estimate their date palms when the fruit ripened, before any of it was eaten. Then he would give the Jews the choice either to keep it according to that estimate or to hand it over to Abdullah bin Rawahah, so that the zakah could be assessed before the fruits were eaten and dispersed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7440