jQuery checks the nodeType property. So you would have:
var overloaded = function (arg) {
if (arg.nodeType) {
// Code for DOM Element argument...
}
};
Although this would detect all DOM objects, not just elements. If you want elements alone, that would be:
var overloaded = function (arg) {
if (arg.nodeType && arg.nodeType == 1) {
// Code for DOM Element argument...
}
};