How to ensure data on old Iphone is completely erased?

Hello,

I do not have access to my old iphone now as I donated to someone to repair and use it. I had a display issue so couldn't clear off the data. Now that person has repaired my iphone and able to use it. I want to ensure all my old data is erased from that Iphone. I removed the device from trusted devices first but now I am trying to delete the data and it no longer shows in "All devices" within icloud. How can I now be sure that all my old data is erased (other than asking that person to do factory reset which I have already asked many times).

Thanks