Look around the table. If you don’t see a sucker, get up, because you’re the sucker.

"Amarillo Slim" Preston (1928-2012) American gambler [Thomas Austin Preston, Jr.]

(Attributed)

Though he used the phrase, he did not take credit for it.  More information here.Variants:

• "If after ten minutes at the poker table you do not know who the patsy is -- you are the patsy."
• "If you sit in on a poker game and don't see a sucker, get up. You're the sucker."
• "If you enter a poker game and you don't see a sucker, get up and leave -- you’re it."