Suggest you google "motherboard form factor".
The best way to identify an ATX motherboard without opening the case is to look at the back of the PC.
All of the expansion cards plug perpendicularly into the motherboard, not into a riser card as with the LPX or NLX motherboards. The other clue is that the I/O ports are in a double row at the end opposite the expansion slots.
The full list of ATX MoBo types - Excluding LPX / NLX and BTX:
Form factor Length Width
ATX 12.0" (30.5 cm) 9.6" (24.4 cm)
Mini ATX 11.2" (28.4 cm) 8.2" (20.8 cm)
Flex ATX 9.0" (22.9 cm) 7.5" (19.1 cm)
Micro ATX 9.6" (24.4 cm) 9.6" (24.4 cm)
ITX 8.5" (21.5 cm) 7.5" (19.1 cm)
Mini-ITX 6.7" (17 cm) 6.7" (17 cm)
Nano-ITX 4.7" (12 cm) 4.7" (12 cm)