I'm Trading In A S7 Phone For An A24 - How Can I Be Sure They Wipe Clean My Trade-In Phone So....

[global1]

they don't leave any trace of my contacts, pics, messages, etc and it's all transferred to the new phone.

I'm told that they'll be able to transfer everything to the new phone.

I never did this before and I'm worried about all my data on the old phone.

Is there anything I should do after this transfer is made to make sure that they deleted all my info off the old phone I'm trading in?

[Timewas]

1. Should be easy

This will erase all data on your device and restore it to factory settings. From the home screen, swipe down from the Notification bar then select the Settings icon > General management > Reset > Factory data reset > RESET > DELETE ALL. Note: A Factory data reset will revert the device back to factory settings.

[comradebillyboy]

2. I always do a factory reset when I trade in an old phone.

That gets rid of everything. Just get your new phone all set up first.

[Wonder Why]

3. You missed something. If, as the OP said,

I'm told that they'll be able to transfer everything to the new phone.

then someone else is doing it and the OP is worried that they will not clean it up.