Which version windows?
There are a few ways you can tell.
Read this: Genuine Windows: frequently asked questions - Microsoft Windows
Its will answer all your questions.
But, if you want to you can do a validation test.
Start > Control Panel > All Control Panel Items > System. Under "Windows Activation" it will tell you if its genuine or not.
Start > My Computer > Help > "Is this copy of windows genuine"
The WGA (Windows Genuine Advantage) is the way to check if its genuine (can be annoying tho)
click validate now on this site and it'll install a active x which checks if your windows is genuine.