Hanumant Adavi has created a comprehensive list of all instances where the Bhagavata Purana quotes, paraphrases, or references other Sanskrit texts, including the Vedas, Upanisads, and Mahabharata. The list proceeds through all 12 skandhas of the Bhagavata Purana in the order of verses. The PDF document [download here] is still in draft form, but it is easily searchable for particular texts (such as “Bhagavad gita”) or verse numbers.