Most motherboard manufacturers list supported CPUs and Memory on their websites.
Others also list this information in the motherboard manuals, which can also be downloaded, before purchase, from their websites.
For example, this Gigabyte Motherboard webpage has links on the right-hand side for CPU Support and Memory Support http://www.gigabyte.com/products/product-page.aspx?pid=3601#ov
Newegg is a good tool (even if you live outside the U.S.). Newegg has "Manufacturer Product Page" links that will help you research your board.