I have been taking lansoprazole 30 mg for six months; how do you know if you still need it?

It depends. On your reason for taking the drug. If you have a condition known as barretts esophagus, it would be inadvisable to stop. If you have gerd, you may want to slowly taper- say take the drug every 2nd or 3rd day and see how you feel. There is an issue with rebound hyper-acidity, so you may want to wait a few days before restarting the drug to be sure that you still need it.