My prefered option is to investigate other modal plugin options such as jqModal. This does expose onShow and onHide event hooks that you can utilise.
Thickbox is showing its age, it does not expose any 'show' events that you can observe.
If you are hell bent on using thickbox then you have a few options.
One option would be to use the global ajax events that jQuery exposes e.g $.ajaxSuccess event. This will fire when thickbox has successfully requested the data to show. However at this point thickbox may not have added the data to the modal, you will have to try this out.
E.g
$.ajaxSuccess( function(evt, request, settings){
//ajax method has completed
});
Be aware that this event will fire on completion of every ajax event that happens within your page therefore you may want some extra checks
Another option is to change the thickbox script and add your own callback code that allows you to pass in a function which will be raised when the modal is shown.