Some clues, and some lists.
Clues can be strength of ink on a older issue,
clarity, sharp lines on circular of the CDS.
Very hard to explain, they just look odd for some reason.
Printed lists of identified fakes such as these:

New (US) Law on Forged Postmarks
Collectors should hail with gladness the interest
the Federal Government is taking in the efforts to
curb faking of stamps which is aimed at defrauding
collectors.
The secret service has promised to go after the
perforation fakers, and recently a law was enacted
by Congress at the urging of Post Master General
Farley, making it a crime to "forge or counterfeit
any postmarking stamp or impression thereof, with
intent to appear that such impression is a genuine
postmarks, or shall make or knowingly use or sell,
or have in possession with intent to appear that
such impression is a genuine postmark, or shall
make or knowingly use or sell, or have in possession
with intent to use or sell, any forged or counter-
feiting stamp die, plate," etc. The penalty is
$1,000 fine or imprisonment for five years, or both.
The Federal authorities are well equipped to cope
with malefactors defrauding the government through
counterfeiting and kindred mischief, but in the past,
as a general rule, they could not be relied upon to
work up any enthusiasm to go after the thieves who
have been preying upon stamp collecting.
However, it now appears that Uncle Sam is much
interested, and a new order is at hand.
We fervently hope so!
- George B. Sloane
Sloane's Column
Stamps
October 12, 1935