Look at the image on this page. It shows a motherboard that has both an inbuilt graphics adapter ( which has a monitor cable connected) and a plug-in graphics card.
The ports on the graphics card are at 90deg to the motherboard. Simplest method for you to determine whether a system has onboard or plug-in graphics is to look on the back panel and see if the graphics port is parallel to the main I/O ports or at 90deg.
This isn't a guaranteed method as I used to have an old compaq system that had a motherboard/daughterboard arrangement. The daughterboard plugged into the motherboard @ 90deg and the graphics card plugged into the daughterboard @ 90deg finishing up parallel to the motherboard. Not common but they do exist.
Goin' Fishin' (Some day)