You should not take a seller’s word for it, including ours. Here is what you can actually verify.
The serial number
Every watch carries a serial engraved on the case, and the same number is printed on the warranty card. Those two must match, and both must match the invoice. Ask for photographs of all three before you pay — any seller who will not send them is telling you something.
The movement
Ask what calibre is inside and check that it is the calibre that reference is supposed to use. A counterfeit will usually have the wrong movement, and the seller will be vague about it.
The finishing
Look at the date window alignment, the sharpness of the applied indices, and the printing on the dial under a loupe. Counterfeit dial text is the tell: slightly soft, slightly wrong in spacing.
The weight
A steel watch has a specific heft. If it feels light in the hand for its size, be suspicious.