See-Through is a transparency spray that you can use to inspect the contents of closed mailing envelopes or paper wrapped packages without opening or damaging them, and without leaving any marks.  

See-Through is a clear, non-conductive liquid that when sprayed on a suspect envelope or document will allow the user to clearly see the contents. After the document or envelope is dry (it dries in minutes), See-Through will not leave any traces or watermarks. Nor does it cause inks to run.