This is why I am not much of a fan of having roommates, you never know if they have a marijuana bush in their closet or the likes...
You can try to do a criminal background check to see if this is maybe a misdemeanor (this cost $$). People have used cirtiphi for their background checks, but this is an area I am unfamiliar with. If you money is an issue or if you are strapped for time (meaning you have secondaries coming), you just have to be honest about it.
It is not usually the misdemeanor that is the issue with the admissions committees, but more the person not mentioning about it. This is where people have their acceptances rescinded (I don't want to scare you). I suggest you search the forums a little bit more as well, read the comments on this tread, and then make a decision. Let's hope this is nothing more than an infraction (then you don't need to report it when ask if you have any misdemeanors) or if you do mention it, that the admissions committee understands this was not your fault.
Good luck.