You will be asked that question during you're interview process. Be completely truthful because you will most likely be given a polygraph examination. But remember, if they only hired people that hadn't smoked marijuana they would have problems filling boots.
I think it depends on how long ago you stopped. If you stopped a week ago... probably not. :) but they can't hold your past against you especially if it was just marijuana. It's like saying you use to have a drinking problem but now you are sober. They can't hold it against you as long as you aren't smoking on the job.