Is there a way to tell if a lazy var has been initialized?

lazy is just a convenience wrapper around one specific lazy-instantiation pattern (and one that is only moderately useful). If you want your own pattern, don't use lazy; just build it yourself.

private var _foo: NSViewController? = nil
var foo: NSViewController {
    if let foo = _foo {
        return foo
    }

    let foo = NSViewController()
    foo.representedObject = self.representedObject
    _foo = foo
    return foo
}

// This can be private or public, as you like (or you don't technically need it)
var isFooLoaded: Bool {
    return _foo != nil
}

override var representedObject: Any? {
    didSet {
        if !isFooLoaded {
            foo.representedObject = representedObject
        }
    }
}

This is designed to follow the isViewLoaded pattern, which addresses the same basic problem.

A shorter version that uses Swift's built-in lazy semantics:

struct Foo {
    lazy var bar: Int = {
        hasBar = true
        return 123
    }()
    private(set) var hasBar = false
}

Just check for hasBar instead.