Not everything what looks different ist a fake, there are actually a lot of printing variances (especially for older cards) and misprints. Below is an example to show you common variances. Please don’t become over paranoid while checking, in worst case you will throw something away what wasn’t bad at all.

This Kaladesh card is actually a simple misprint, the black (key) layer was printed twice, so it is off and looks weird.

There are multiple nice resources regarding misprints on the web. One of them is misprintedmtg.com, they have a nice beginners guide to misprints aswell.