Trading in my old iPhone how do I know all my information is deleted followed all the steps just want to make sure

If you followed ALL the steps in this article, your data is gone from the phone --> What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

If you went through the Erase All Content and Settings and let it complete, you should be fine. When you switch on the phone you should be met with the Hello screen scrolling in different languages - just as it was when it was fresh out of the box.