First year using sensors. Any advice?

[Nirnaeth]

Our team has enough depth in terms of mentors and students that we'd like to add some sensors to our robot. Since we've literally never added a single sensor to any of our robots, I was wondering if we could solicit some advice.

Our specific application: We have an arm that wants to rotate back and forth through a 90-degree turn. The arm is connected to a window motor. What manner of sensor should we use? I'm assuming some type of rotary encoder?

Thanks!