Here is a longer list including the items above:
Does the
style match the expectations for the
type? (Strange
style may mean pressed or struck with modern dies.)
Does the metal match expectations for the
type? (Many replicas are made in a different metal than the genuine original.)
Does the
fabric match expectations for the
type (thick, thin, squared, oval, round, etc.)?
Does this coin have the expected edge for the
type (ragged, with or without sprues, beveled, straight, etc.)?
Does the strike match expectations for the
type (weak, strong, even, uneven, centered, off center)? (Perfect on a
type that is almost never perfect needs close examination.)
Do the flow lines match expectations for the
type with similar die wear? (Particularly for some
Roman silver, the more apparent die wear, the stronger the flow lines should be.)
Does it have edge cracks and splits within normal expectations for the
type? (Too few for the
type may indicate casting. Too many for the
type may be a sign of machine pressing.)
Does the
weight fall within expectations for the
type? (Some
types are never light. Some often are, but underweight is
still a reason for a closer look.)
Does it have casting bubbles?
Does it have casting pearls from bubbles in a mold created by casting?
Does it have a ghost line indicating the edge of an original coin used to make the mold?
Does it have missing
legend or parts of the
type and indications that it was cost from a mold made with a coin with those parts off
flan?
Does it have waves in the fields (from uneven cooling after casting)?
Are the fields unusually flat? (May indicate machine pressing)
Does it have filing on the edge?
Does it have a casting or
electrotype seam on the edge?
Are the edge cracks and splits real?
Does it have tooling?
Does it have a natural
patina or an artificial
patina? (A genuine coin may have an "
enhanced"
patina, but a fake coin will never have a natural
patina.)
Does it match any fake in the
fake coin reports?
Is the
price especially low? If it is on
auction, are others not bidding it up to the normal
price for the type/quality?
Is the seller also listing other fake or questionable coins (if you suspect one coin, look at the rest of the offerings).
Is it for listed on
eBay by a member of the
Notorious Fake Seller List (
NFSL)?
Is it for sale on
eBay and the seller says
no returns or has private listings?
Is it for sale on
eBay and it is a very
rare type or emperor, misattributed as a common coin?
The challenge of authentication is not just knowing what to look for generally, but also knowing what is normal for the particular
type.
When
NGC says they do not guarantee authenticity, what they mean is they aren't going to give you any
money if they are wrong. How could they? You pay them $45 and they are going to guarantee your $5000 coin or even your $500 coin is genuine? Really they don't guarantee grading either.
NGC authenticates coins and they have expertise. The guarantee limitation is not even worth mentioning.
NGC is an unbiased highly expert opinion worthy of considerable respect. They can be wrong but they are more likely to be right than you are (and you means almost everyone).