Well, that depends on what OS do you have. You could go to control panel in windows and see microphone settings if you can test your microphone or something like that. Or check the your device manager if you can see the microphone there.
Go to Accessories in your Start Menu... Then go to sound recorder... and say something when you start it. If it can record then you got a microphone or else no. You can also try calling someone from your PC and know about it.
Make a call to your friends and speak with them. You can confirm whether your laptop is having mic or not.
So far for desktop computer there is no built-in microphone. Even the known once and branded ones do not have a microphone with them built-in or not. Only Laptop do have microphone embedded inside the laptop. But if really want to verify it, i suggest you follow all the previews post before mine to check for a microphone in your desktop computer. But if your computer is a laptop then it should have a built-in mic and to very this try to record your voice by going to the sound recorder as suggested on the previews post and watch the level meter to see if there is activity while you speak during recording or simply after recording then play it to hear you voice to verify the microphone existence in laptop computers.