simple test

When you shut down your computer does it power off on its own, or do you have to hit the power button? If it powers off on its own you have an ATX if not then it's an AT. Also, if the power switch is a momentary switch then ATX, if it's a switch that clicks on then it's an AT. If those tests are inconclusive then look at the mobo connector from the PS. An ATX has two rows of 10 wires, and an AT has a single row of wires made up of two cable ends with the black wires from each next to eachother. Hope this was clear enough.