Several ways to tell:
Often, when there is an "M" after the model (ex: P8H61-
M LX2), I've found that the board is often a microATX board. That's only an indicator, do further research in case that standard doesn't always hold true.
The only surefire way is to look at the spec sheet of the board itself. Your motherboard (according to its specifications sheet) has the microATX form factor (µATX or more commonly, uATX). Seen
here. (scroll to the bottom, look for the "Form Factor" secton)
The picture of your motherboard (on the manufacturer's web-page) is misleading, because it really looks more like an ATX board there. (I trust specs more)
I hope that helps!