How can I know my Lenovo laptop model?

For ThinkPad, the product name is printed on the LCD Bezel of your machine.
For IdeaPad and Lenovo notebooks, the model (product name) is printed on the label at the bottom of your machine.

http://support.lenovo.com/sg/en/find-product-name

LENOVO YOGA 320/520/720/920 Series - 14 inch

LENOVO YOGA 320/520 Series - 15 inch