How to tell which websites take your MAC address of your iPhone/iPad when visiting? how can you tell when viewing webpages, which ones are taking down your device info like MAC address, when you use an iphone or ipad?
Or how to prevent that information being taken?
It isn't possible for a website to get the MAC address from an iPhone or iPad.

For that to work, Apple would have to expose that information through JavaScript. They haven't done that. Don't take my word for it though, try a web search for something like "javascript get ios mac address" and you'll find lots of people asking how to do it and lots of people replying that it isn't possible (except in Internet Explorer on Windows in some very specific conditions).

Years ago you could get the MAC address of an iPad or iPhone if you wrote an actual iOS application. However Apple changed that in 2013 with iOS 7 as detailed here.