Most of what I’ve seen involves using Infrared sensors. Usually IR Reflective Sensors, or beam break sensors (like the one @Lexeo linked). The two I have been looking at are the SHARP GP2Y0A21YK0F IR Reflective Analog Distance sensor, and some generic Infrared Photoelectric adjustable trigger distance digital sensors.
Here are the specific listings I have bought (but haven’t actually experimented with either yet).
SHARP Analog Sensor: https://www.amazon.com/dp/B00IMOSEJA/
Photoelectric Digital Sensor: https://www.amazon.com/dp/B00M1PN0S6/
For 3468, it sounds like we are planning on putting a sensor on each end of our conveyor system. The one at the intake will be used to detect a ball coming in and make sure it gets moved up enough for the next ball to come in, and the one at the top being used to stop any more from being taken in to keep any jamming from happening if the driver losses track.