It's important to interpret passages of scripture acording to the intended context and audience of the author. In this case, the audience was first century jews wh understood symbology (especially when coming from a person who wrote it in prison) A jew referring to one's hand and/or forehead would be understood to refer to a person's thoughts and deeds. In that context, the passages is saying that followers of the Beast will be known to others by what they say and do. For that reason, it's reasonable to interpret this passage as the mark being a spiritual mark, not a literal mark.The bible says it will be on the forehead or hand, leads me to beleive it will be visiable, what it is I don't know, so far, I don't think it really tells us what it might be.