JOEL - Someone else has seen the plates. Joseph Smith did show them to at least eleven other people who handled them and observed the writings on them.
However, for the rest of us the best proof for the Book of Mormon is the Book of Mormon itself. Anyone who reads it with an open mind and the proper atitude cannot help but realize that it is of divine origin. The difficult part is getting people to actually read what they are criticizing.
There will never be enough scientific proof to prove the authenticity of the Book of Mormon. Even if we had the gold plates, no linguist would be able to translate them; no one would believe that an angel gave them to Joseph Smith; and no one would believe he translated them through the power of God. One of the main purposes of the Book of Mormon is to cause people to exercise faith in believing what it says and in what it teaches us. If we had proof, there would no longer be any reason to exercise faith in it.
By the way, even though there are some manuscripts that exist from which the Bible was written, there is still no proof that what we read in the Bible actually happened. To many people the stories are just legends that were told to teach a lesson but that never actually occured. We still need faith to believe in what the Bible tells us as well, otherwise it is just another book of interesting stories.
Just because someone wrote something down a long time ago and called it scripture, dosen't mean that it is true. The Holy Spirit has to witness to us that it is true. In this regard, you might ask your critics how they know what is written in the Bible is true and something that actually happened. For this is really the most important question. They will probably say something like, "I have read it and studied it and have applied its teachings in my life and the Spirit has born witness to me that it is true." Then you can tell them the same thing about the Book of Mormon and the Bible. Then challenge them to actually read the book and pray about it and then leave it at that.