How can I find out how a country has voted in the General Assembly?

The following information comes from the UN Voting research guide:

Information about votes in the General Assembly can be retrieved through the United Nations Digital Library. Here you may search by resolution symbol, keyword and the date of voting. It is not possible to search by country. 

To find how a country voted, limit the search selecting Vote in the facets in the left side. 

From the search results page, click on the title of the resolution to see the full vote information. 

The Index to Proceedings of the General Assembly (ITP) contains a Voting Chart of resolutions adopted, indicating the vote of each member state. Consult this to find out how a country voted during a session of the General Assembly.