If you're talking about the standard office desktop case and mobo, not any of the recent "entertainment PC" solutions, there's no way to tell this, unless some of the cards on a standard ATX mobo happen to have been installed in some of the furthest slots, which are not present in the short micro-ATX mobo format.
The other way would be to hit the Pause/Break key at the very first bootup screen, freezing it, and see if any pieces of useful information re the mobo type are shown on the screen and go googling on those bits of info.
What were you aiming at btw?