Eventually The HoF will have to start letting in some players accused of using steroids in, or as Dave said there will be no inductions for 10 or 20 years. With some players it is fairly obvious when they started using roids, primarily late in their careers. I think voters will eventually look at their numbers up until the point they are suspected of using roids and decide whether the pre-roids numbers justify entry into the hall.
The question is, how do you know they weren't using the entire time?