How you know how deep a metal detector can detect objects?

Ive been looking to buy a metal detector online but every website I went I couldnt find how deep the detector can go to detect the objects, how can I know this is a certain code in the details or something?

In general an average metal detector can detect coins upto 8 inches or so. A very good one, such as the Minelab Explorer has been know to find them upto 12 inches or so depending on the size of the coin and type of soil.

A very cheap detector (less than $100 new) will only be able to tell you if there is metal in the ground and only a few inches. A better detector coin penetrates further and can discriminate between coins and iron so you spend a lot less time digging and a lot more time finding. A very good detector, with experience, can give you an idea of what kind of coin is buried.

Accurate Underground Metal Detectors 3D Imaging System