I can assure you that one sip is enough for me to reliably identify it. Both Diet and Zero leave a very distinct and unpleasant aftertaste that's just absent on the regular one.

(doesn't apply to Pepsi though - here the regular one also tastes bad now as it contains sweeteners as well, so I doubt I'd tell it apart from Pepsi Max unless trained beforehand - they're both gross, so I unfortunately had to switch to Coca-Cola for my occasional coke cravings)