How can I determine if a servo motor is in the commanded position?
You should take a look at the Analog Feedback Servos.

Here is a resource : About Servos and Feedback

And if you want to modify a classical servo and get the feedback from it, here's an article about how to hack a classical one. The hack is really easy to realize and your understanding of servos will be better (as all good hacks).

Basically, an analog feedback servos (buy as-is or hacked) will allow you to read from the servo the current position.