Witness: FTX had $5 billion in crypto at market top, should have had $20 billion
FTX Was Supposed to Have $20 Billion in Customer Crypto—It Only Held $5 Billion
An expert witness called in Sam Bankman-Fried's trial said between January 2021 and October 2022 FTX never had all its customer's crypto funds.
https://decrypt.co/202223/ftx-had-5-billion-when-it-was-supposed-to-have-20-billion
At the end of the day, the only way to know for sure if an exchange actually has assets on its books is to withdraw them to your self-custody.