The key word in your question is "suspected" - that would make me hesitate to call the police. As druhepkins stated below if you're suspicions are wrong it can destroy a person or family's life.
I would keep my antenna up though and start a journal documenting dates, people, cars, and suspicous activity. Build a case and then if the evidence starts to mount you may want to talk to the police.
If you can present them with some evidence they might start their own stakeout or investigation.
It's a tough position you find yourself in, especially with something like Meth.
I hope your suspicions are wrong, but if not be ready to make the right decision.