It seems that a lot of people have a problem with this rule. In my group, where there is a forsaker in one of our campaigns, we simply removed the magic destruction ability and made it so that the forsaker had to somehow "sacrifice" a particular amount of personal wealth every week (a day is too short, we thought) in order to keep his DR. It doesn't seem to be unbalancing yet, but we just made the rule so we'll have to see how it goes.
However, if you're intent on keeping it in there (which is perfectly reasonable, don't get me wrong), there are a few things you can do about it, I think, to make the adjudication easier. First of all, an idea that the forsaker in our group had before we changed the magic destruction ability was to carry around potions in small vials (the vials would be dusted with lead, too, so that he wouldn't be subject to a detect magic spell, because he didn't like the idea of having to carry around magic to be effective but that's pretty much the only way it works with the rules as written), and then before the big battle of the day he would throw the vial down to the ground, breaking it and ruining the potion. We ruled this would be a move-equivalent action, since it should take less time than drinking a potion (standard action) but doesn't seem quite right for a free action. There were also ideas with lead scroll cases with some sort of blade inside that was attached to a trigger that would cut the scroll or something, but that idea wasn't developed enough before we changed the rules. So, if you don't have a problem with your forsaker, then you can try those ideas.
Also, I would probably rule that if a forsaker wanted to destroy magic, even if they didn't have the DR at the moment, it would be okay to treat his hands (or weapon) as +x weapons for the purpose of destroying magic items...after all, "magic destruction" is an ability, so it should let them be slightly better at it than most people, somehow. Either that or I would make them spend a standard or full-round action to "destroy" a magic item, amybe the magic item would get some sort of save or something, but they would have a good chance of succeeding as long as it wasn't something really important (like an artifact or something, obviously).