Last year we used a digital Banner sensor with a trim adjustment wheel to detect the gaffer's tape for our non-low bar 2-ball auto mode. Initially we tried to detect the green auto line tape (worked great in our lab with what we thought was the same tape), but at the Championship event we changed it to sense the white mid-line tape instead. The latter worked really well. Don't try to detect green on green
Some tips for doing this:
1) Many optical or IR sensors do the job, but nice features to have are a trim wheel and an LED to tell you when you do/don't see the tape. Better sensors also tell you when you are close to the threshold (e.g. you see the tape, but barely - keep tuning for robustness).
2) The sensors we used worked best a few inches above the carpet. Each sensor is different, but most of them will have a range of heights where they work well.