Another thing you may be able to try is to check the event that gets passed to checkAddress, it has lots of useful properties for things like this. For example:
on(document.getElementById('ownerAddress'), 'focusout', checkAddress);
function checkAddress (event) {
console.dir(event);
// event.target or event.srcElement will both point to the node that triggered the event, a.k.a. ownerAddress
// There are also a lot of other properties attached to event that can also be useful
}
In this case you don't have to do anything special to get event into checkAddress, it is automatically passed in for you (unless you use Angular).