How to check a struct exists at a particular memory address?

[Gary Willoughby via Digitalmars-d-learn]

This might be a really silly question but:

I've allocated some memory like this (Foo is a struct):
    this._data = cast(Foo*) calloc(n, Foo.sizeof);

How can I then later check that there is a valid Foo at 
`this._data` or `this._data + n`?