how can i be sure my old iphone is erased

Hello, yes when you erase all content and settings that means your old iPhone is wiped, and as you saw it shows the hello screen as you were setting up a new iPhone that is what you want, the only reason it showed if you would like to transfer the info from the new iPhone to the old iPhone as you said it is because they are close by.

Yes you did. Erasing the phone and getting the Hello screen takes care of it.

If you have any doubt just look at this article: https://support.apple.com/en-us/HT201351

This might help re-assure you: Set up the phone as a new user - completely different from your old user account on that phone. Load it with enough data to completely over-write any data that may have remained from the previous account.

Then erase it again, following the steps you’ve already used.

danadeee wrote:

I had to get a new iphone, because both front and back of old phone were broken. I backed the old one up and transferred my info to the new device. Removed SIM card and added it to new device, and restored from icloud. But for insurance reasons I have to send my old phone back in. I want to be absolutely positive that all my information, notes, photos, and contacts are off the old phone. I turned off find my iphone, I signed out of my apple ID, and I hit "Reset" and then "erase all content and settings" ...the old phone then turned off and I turned it back on to check it, and it took me to the "hello" screen you see when setting up a new phone. But then it asked me if I would like to transfer the info from my new phone onto the old one, because it was close by. I just want to be sure that the old phone is wiped... did I do everything right? thanks

Yes you did it right and if you want more confirmation read here What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

danadeee wrote:

Thank you!

My pleasure. And BTW your question was very well stated. I wish other users posted a thorough question like you did.