Authenticity cannot be settled on a video call. In this category the more common problem is not a wholly fake item but a real one with replaced parts, which is a question you can ask about directly.
What to ask them to show
The joints worked, on camera. Ask them to move it. Tightness and how it sits is something you can see happening and cannot see in a photograph.
The paint, close up, on a raised edge and in a recess.
Whether any part has been replaced, asked as a direct question. "Is everything on this original to the piece?" is a question an honest seller answers plainly and a careless one answers vaguely.
The box, all corners and both long faces — for a boxed collectible the box is most of the value, and crushed corners are the commonest disappointment here.
Any maker's marks, photographed rather than described.
What a refusal means
"I think it is all original" is an honest answer and tells you to price accordingly. "It is 100% original, no question" with no photograph of the markings is reassurance.
A seller who will not move the joints while live is odd, because the item is in their hands.
What this page will not do
It will not tell you what original paint looks like against a touch-up, or which parts are commonly reproduced for which piece. That knowledge is specific, it changes, and a general version of it is how a buyer talks themselves into a purchase.
What protects you if you get it wrong
A reproduction sold as original is not as described — returnable for 7 days whatever the store's policy, and reportable if it was deliberate.
Ask how the box will be packed, too. A box posted as the shipping carton is the single most common way value is lost in this category, and it is not a fake problem at all.
Questions people ask
Is a restored piece a fake?
No, but it is a different object at a different price, and it must be described as restored.
What if the seller does not know?
That is common and fine, as long as they say so. "I do not know" lets you price the uncertainty.
Does the box really matter that much?
For a boxed collectible, often more than the item. Ask for photographs of it before you bid, not after.
More: spotting a counterfeit before you bid, Metzee's position on counterfeits, reporting a seller, questions to ask before bidding on toys and collectibles.