a) If it seems
possible that he has been to NYC (he talks like a man who has indeed been to NYC).
b) If it's
impossible (counterfactual) that he has been to NYC.
Mark Israell says:
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The
past subjunctive is used:
(1) for counterfactual conditionals: "If I were..." or
(literary) "Were I...". In informal English, substitution of
the past indicative form ("If I was...") is common. But note
that speakers who make this substitution are *still*
distinguishing possible conditions from counterfactual ones,
by a change of tense:
Present
Past
Possible
condition:
"If I
am" "If I was"
Counterfactual condition: "If I were/was" "If I had been"
...
"As if" and "as though" were originally always used to introduce
counterfactuals, but are now often used in "looks as if",
"sounds as though", etc., to introduce things that the speaker
actually believes ("It looks as if" = "It appears that").
In
such cases the present indicative is often used. ("As if" and
"as though" are exceptions to the above table in that they take
the
past subjunctive, not the
pluperfect subjunctive, for
counterfactuals in the past. The past tense of "If he were a
fool, he would mention it" is "If he had been a fool, he would
have mentioned it"; but the past tense of "He talks as if he
were a fool" is "He talked as if he were a fool." "He talked as
if
he had been a fool" would mean that he seemed, not foolish,
but regretful of
earlier foolishness.)
...
http://alt-usage-english.org/excerpts/fxsubjun.html
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Along these lines (if he is NOT a New Yorker):
He talks as if he were a New Yorker.
He talked as if he were a New Yorker.