Suppose you're the co-pilot on a flight and during pre-flight, talking to the captain, you smell alcohol.
Although you didn't see him drinking and the smell isn't strong, what does regulation stipulate you should do?
Unfortunately, at least in the US, the regulations don't stipulate that you do anything. Airline policy is a different matter, and one which I cannot speak to, as it will be different in every case.
I can provide some background and my own take on this, but it's hard to have one exact answer (what if you're wrong?).
FAA regulations rely on pilots self-regulating. If a pilot drinks within an 8 hour (or, as is often policy for airlines, 12 hour) window prior to the flight, they are required to remove themselves from that flight. If they do not take this action, somebody else must. Often, this person is the First Officer, a junior pilot who has years of flying with pilots who might come to view them as untrustworthy. Perhaps this can help explain why it's such a difficult action to take.
This is a difficult topic, and one which regularly comes up during airline flight training. That said, the courses I took that discussed this scenario essentially landed on a couple of steps (this was years ago, this is probably a bit rusty):
Beyond that, I don't want to make suggestions. Getting them to talk to the union they are a member of is probably a wise idea, as they can offer possible solutions or arrange treatment if the pilot has alcohol abuse problems (which is not a huge stretch, if they're showing up to work reeking of booze). AOPA Flight Training has a blog post that discusses some similar scenarios.
This is clear cut. You are required to wait 8 hours after taking a drink of alcohol before you fly a plane. According to FAA regulations:
(a) No person may act or attempt to act as a crewmember of a civil aircraft—
(1) Within 8 hours after the consumption of any alcoholic beverage;
(2) While under the influence of alcohol;
(3) While using any drug that affects the person's faculties in any way contrary to safety; or
(4) While having an alcohol concentration of 0.04 or greater in a blood or breath specimen. Alcohol concentration means grams of alcohol per deciliter of blood or grams of alcohol per 210 liters of breath.
(b) Except in an emergency, no pilot of a civil aircraft may allow a person who appears to be intoxicated or who demonstrates by manner or physical indications that the individual is under the influence of drugs (except a medical patient under proper care) to be carried in that aircraft.
(c) A crewmember shall do the following:
(1) On request of a law enforcement officer, submit to a test to indicate the alcohol concentration in the blood or breath, when—
(i) The law enforcement officer is authorized under State or local law to conduct the test or to have the test conducted; and
(ii) The law enforcement officer is requesting submission to the test to investigate a suspected violation of State or local law governing the same or substantially similar conduct prohibited by paragraph (a)(1), (a)(2), or (a)(4) of this section.