It is not a good sign for someone to use the words "whatever it takes," when describing their readiness to stop using. You would think that is precisely the attitude you would want a user to take, but somehow that particular phrase is a red flag. It will be replaced in 24-48 hours by some hedging about how this rehab thing is going to play out in practical reality.
There is the Goldilocks version of this growing avoidance: That rehab is too far. That rehab is too near. No rehab is just right.
For 80"s rockers, there is the Meatloaf version: "I would do anything for rehab," said with firmness and intensity on Monday morning. "But I won't do that," said Tuesday afternoon. It would be one thing if these were people who were suspicious whether rehab does much good, as I am. But this is from the rehab-immersed culture, with spouses, siblings, and friends all familiar with the various advantages and disadvantages of each program.
Believe it or not, wearing clothing with a beer logo on it to your rehab interview is often interpreted by others as an indication that you're not serious. Imagine that.
If someone claims to be clean but has a positive blood or urine sample, multiply the amount they eventually acknowledge using times six to get a more accurate picture.
Ask specifically about marijuana, as many users pretend that's not really a drug.
Heavy drinkers tend not to wear their seat belts when sober, but might wear them when drunk. I don't fully understand this, but there may be some balance of risk involved. Notice that other people's risk does not factor into the equation.