The light test is one of the most common and most important tests to know and it is easy to perform. You need a strong light source, i.e. a mobile phone light or LED flashlight and shine through the card. If you see the wave pattern of the inner layer and it has a blueish shine, your card passed the test.
The blueish shine comes from the blue core of the “Corona” paper used by Cartamundi to print the cards. You can read more about it here.
But beware, there were actual misprints in recent sets (Ixalan) which were printed on the wrong cardstrock. These misprints will fail the light test, but it is an uncommon issue. You can read more about it here.