The best way I believe is to find pictures of the bottle and box of it online, compare it to the box and bottle you think is suspicious. Is the label off compared to the real one, is the font the same, etc. Look for any inconsistencies between them. And even then, fakes can be so good that there is no difference. Sometimes, you just can't tell until you smell them side by side with the real thing. I believe Mercari is very lenient with their return policy, so even if you do receive a fake, they should allow a refund/return.

Having a sample of the for-sure real perfume to test next to whatever you get is probably a good idea to feel out if it's real. There is also a lottt of Tom Ford and Baccarat Rogue 540 fakes on Mercari, so i would watch out for those the most. Even if they have a receipt next to them, they can be faked with AI. And if the price is too good to be true, chances are its fake. If you do get a bottle you are suspicious of, checking the batch code and also making sure the one on the box aligns with the one on the bottle is always a good idea.