@kalday1989,
bruegel did lots of his masterworks (Gift of the Magi and the grissaille painting of "Christ and the Woman taken in Adultery" ) as watercolors on paper. His "Day of ST Martins was a tempera on BOARD . Bruegel was best known as a printer/engraver so most of his works were known to be papaerworks. Youd want to make sure that the paper is really "old". For it to be so it would be "laid papaer" where you can see the layers criss crossing in transmitted light. Laid papaer looks amazingly like a weave (it sorta was) of the linters rather than a pressed batch of pulp. SO
1does the paint look like watercolor? or tempera?
2Does the paper look like laid paper?
3Does the whole thing exhude a certain oldness?
Id then have it looked at by a reputable appraiser /conservator
What the hell, a primary copy of the Declaration of Independence was found being used as backing in a picture frame in a flea Market in Pa. The last time this copy sold at auction it went for 15 million bucks