Authenticity cannot be settled on a video call. For clothing this is especially true: the details people rely on change with every production run, and a counterfeit made this year is built to survive last year's advice.
What to ask them to show
The main label, close and in focus.
The care and composition label, which is usually inside a side seam and is the one sellers forget to photograph.
Any hardware — zips, buttons, rivets — close up, and ask them to run a zip.
The inside seams, turned out if the garment allows.
The garment laid flat with measurements taken on camera: chest across, length from the shoulder, sleeve. A stated size is not a measurement, and this is the request that prevents the commonest disappointment in this category, which has nothing to do with fakes.
What a refusal means
Photographing a care label takes ten seconds. A seller describing a garment as a particular brand who will not photograph its labels has declined the simplest possible request.
"It is authentic, I have had it for years" is not a reply to "may I see the inside label?"
What this page will not do
It will not tell you what any brand's labels or stitching ought to look like. The plan for this site marks this category as needing somebody who follows it closely, and that is honest — written from general knowledge, such a list would be out of date and dangerous rather than merely useless.
So: gather the photographs, and if the garment is expensive, show them to somebody who buys that label regularly before you bid.
What protects you if you get it wrong
A counterfeit is not as described — returnable for 7 days whatever the store's policy says — and it is reportable.
Questions people ask
Is second-hand clothing risky to buy?
Mostly the risk is fit, not authenticity, which is why the measurements matter more than anything else on this page.
Does a label prove anything?
Labels are the easiest thing to copy. A photograph is still evidence worth having.
What about something sold as inspired by a brand?
That is a different item and must be described as one. A replica sold as a replica is allowed; sold as genuine it is not.
More: spotting a counterfeit before you bid, Metzee's position on counterfeits, reporting a seller, questions to ask before bidding on menswear.