Play poker for significant amounts of money. While it only tests limited and specific areas of rationality, and of course requires some significant domain-specific knowledge, poker is an excellent rationality test. The main difficulty of playing the game well, once one understands the basic strategy, is in how amazingly well it evokes and then punishes our irrational natures. Difficulties updating (believing the improbable when new information comes in), loss aversion, takeover by the limbic system (anger / jealousy / revenge / etc), lots of aspects that it tests.