Knockout 2.0 has two functions that you could use for this purpose. ko.dataFor and ko.contextFor take in a node and return either the Knockout data that would be available to it for binding at that scope (dataFor) or the entire binding context at that scope (contextFor), which includes $data, $parent, $parents, and $root.
So, you can do something like:
var isBound = function(id) {
return !!ko.dataFor(document.getElementById(id));
};
Here is a sample: http://jsfiddle.net/rniemeyer/GaqGY/
However, in a normal scenario you should really look to call ko.applyBindings a single time on your page. It depends on what you are trying to accomplish though. Take a look at this answer for some suggestions on ways to manage multiple view models: Example of knockoutjs pattern for multi-view applications.
Typically, you would do a <div data-bind="with: mySubModel"> and when mySubModel gets populated then that area would appear and be bound.