Go to control panel > Sounds and Audio Devices > Audio tab. If it says "Realtek", then its onboard audio. If it says something else, it must be a sound card.
There might be other onboard solutions apart from realtek. Not sure of that.
u can also check to see if there are two audio jacks on the back panel - one green and one red (this is common in comps nowdays, but in old comps it was just 2 silver audio jacks)
check ur mobo manual for the sound chipset used if u have onboard sound.