A lot would depend on the price. If it is a "you stole it" price, you wouldn't have to be that fussy. If it is a "get a second mortgage on the house" price, you need to research it.
As Fred said, there are two types. I also know that they were usually serialed to the rifle (i.e. the rifle's s/n was stamped on the scope). For starters, if you have M1903 references, like Brophy, Campbell and Canfield, start reading!
"We make men without chests and expect from them virtue and enterprise. We laugh at honor and are shocked to find traitors in our midst."
--C.S. Lewis