They are two easy ways to identify the difference between an AT and ATX power supply.
An AT motherboard power connector is two plastic blocks, each with six wires in it. The black wires must face each other. click here
An ATX motherboard power connector is one plastic block of 2 rows with 10 pins on each row. click here
The other way is how the case power switch is connected. If it has two thin wires going to the motherboard, it is ATX. If it has a thick power cable running to the PSU, then it is AT ( as it carries mains power ).