How to know if it's safe to erase all my content from my old iPhone?

I've had an iphone for about 2 years and just upgraded it yesterday. I figured since my mom still has a brick phone I would give it to her. I wanted to erase all the content off my old one before giving it to her, but I want to make sure I don't lose everything. I backed everything up on icloud, but that's it. All my pictures, apps, ect. Are now on both devices. Is it safe to reset the old phone, or are there more steps I have to take?

If you backed everything up into icloud and it's on your new phone, you can restore your old iphone to factory settings and you won't lose anything.

Make sure you have a backup of the old phone, then restore the new phone from the backup of the old phone. Once that has been done successfully, you can reset the old phone. You can use iCloud backups, but it works best if you use encrypted iTunes backups on your PC or Mac.