First off, who did you get it from and how much did you pay? Those two things are the biggest indicators.
Next, you have to look for design flaws. People say "Oh, make sure it has a sticker, make sure it has a label" - this stuff means NOTHING. You think a company who isn't paying licensing fees is going to go online and find a company to replicate a sticker and they pay $120 USD for a roll of 1200 high quality stickers that increase the chances they can sell these things? When people make knock off stuff now a days, they don't just look at a picture of it. They buy the real thing, take it apart, make a pattern out of it and then develop an assembly line and order the materials. The only difference between a knock off and a genuine product is the quality used to manufacture the goods. If a company sews a bunch of crooked lines on a brim of a snap back - nobody would buy it. So a company that makes quality products destroy the product (like Prada, Versace, Armani, etc) or its sold in liquidation and even then the product isn't considered genuine because of its design flaws.
Don't get confused though with things like normal error stitching. This is how someone determines a luxury hang bag was made by a skilled person and when it was made by a machine. A machine will reproduce the same movement 99% identically as the movement before it. A human can not, even if they are extremely skilled but they can come pretty close.
Here's a sign something is fake. "Smuggled out of actual factory" - factories look at products like you look at dollar bills. Where you might see 1290 hats sitting in a big bin in the corner of a factory, they see $11,600 USD in cash. Kind of like drugs. Middle management keeps a keen eye on their works to impress upper management and to keep their jobs.