How can you tell if a "player" is being sincere or just hitting up another "mark"?

This is a follow up to the question of whether or not guys play hard to get -- the hot-and-cold game that's basically designed to keep a gal on the hook. (I guess that bc I'm used to dealing with guys who are linear in indicating that they're interested or they're not it's odd to see a guy be distant then suddenly shift gears to intense interest... then back to zero attention again.)

Presuming this is some move in the "player's handbook" how would a gal know whether a player's actually being sincere or just trying to keep her on the hook along with a string of other gals? Or should a gal just assume that "once a player always a player"?

How can you tell if a "player" is being sincere or just hitting up another "mark"?
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