How to detect if a variable is an array

What is the best de-facto standard cross-browser method to determine if a variable in JavaScript is an array or not?

Searching the web there are a number of different suggestions, some good and quite a few invalid.

For example, the following is a basic approach:

function isArray(obj) {
    return (obj && obj.length);
}

However, note what happens if the array is empty, or obj actually is not an array but implements a length property, etc.

So which implementation is the best in terms of actually working, being cross-browser and still perform efficiently?