Luckily it is pretty easy to find out. One way to do it would just be to replace the hard drive in your computer saving away your old one putting a new one in installing Windows 8.1 and seeing what happens,
On the other hand you can check ahead of time. The Windows Compatibility Center on Microsoft website will give you information by testing your computer to see what it contains and to see if it will support Windows 8.1.
It's worth checking out the Microsoft site and search for "Windows compatibility Center".
Windows Compatibility Center: http://www.microsoft.com/en-us/windows/compatibility/CompatCenter/Home