If you are looking for a will, you can search the Nationaal Archief's Central Register of Wills (CTR). This register contains all wills that were drawn up between 1890 and 1973. The CTR can be used to find out whether or not a will was drawn up and if so when this took place and before which notary.
Until about 1950 naturalisation, or the granting of Dutch citizenship to foreign nationals permanently settled in the Netherlands or the Dutch colonies, was not a common phenomenon in Dutch history. A foreigner was permitted to live and work in the Netherlands as long as he was self-sufficient and not a nuisance to anyone.
At the Nationaal Archief you can find the Royal Decrees in the archives of the Secretary of State to King William I (1813-1840) and in the archives of the King's/Queen's Cabinet (1841-1988). These archives are what is known as the verbaal archives and are ordered chronologically according to the date of the decree.