Bought a pair of PreSonus headphones at a C&M store (they also sell guitars and all that stuff) but I can't tell if it's ori or a fake like all the fake Beats by Dr. Dre sold in usual mall shops. I usually check for spelling errors on the box but nothing looks out of the ordinary here. It sounds okay but cant know for sure.

Try search online if there's people that compare real vs fake.

Likely they are not fake. Cloner like to fake famous brand name because they are easy to sell - and PreSonus is not considered as 'famous' when compared to Beat or Sennheiser.

got diedar or diimport sticker?

Is there an email address that you can send email direct to the manufacturer?
You can try send email with a snap shot photo of the box (with serial number) and ask them to check whether it is genuine product.
I tried that before and I did receive reply to confirm that the product I bought is genuine.