Mo. Rev. Stat. § 115.259: Voting machines to be visible to election judges at polls.

At each polling place using voting machines, the exterior of the voting machines shall be in plain view of the election judges. The election judges shall not be nor permit any other person to be in any position, or near any position, that enables them to see how any voter votes or has voted. The election judges may inspect any machine as necessary to make sure the ballot label is in its proper place and that the machine has not been damaged.