In Javascript. how can I tell if a field exists inside an object?

This will ignore attributes passed down through the prototype chain.

if(obj.hasOwnProperty('field'))
{
    // Do something
}

In addition to the above, you can use following way:

if(obj.myProperty !== undefined) {
}

UPDATE: use the hasOwnProperty method as Gary Chambers suggests. The solution below will work, but it's considered best practice to use hasOwnProperty.

if ('field' in obj) {
}