怎样区分that做指示代词还是引导定语从句Geneticengineeringintroducesintofoodgenesthatarenotpresentnaturally,cannotbeintroducedthroughconventionalbreedingandmayhaveunknownhealtheffectsthatshouldbeinvestigated

　　怎样区分that做指示代词还是引导定语从句

　　Geneticengineeringintroducesintofoodgenesthatarenotpresentnaturally,cannotbeintroducedthroughconventionalbreedingandmayhaveunknownhealtheffectsthatshouldbeinvestigatedbeforethefoodissoldtothepublic.句子里面的最后一个that参考书中说是指示代词,指代foodgenes,不明白为什么不是做effects的定语,怎样区分呢