Since 1 June of this year, if a sample tests positive for clenbuterol, but the concentration is below 5ng/ml, laboratories report an atypical finding (ATF) instead of an adverse finding (AAF).
In case of an adverse finding, the anti-doping authority essentialy has to charge the athlete with an anti-doping rule violation, while an ATF allows for an investigation before making the decision on whether to bring charges or to close as "no case to answer".
When a sample tests positive for clenbuterol but the concentration is less than 5 ng/ml, the responsible anti-doping authority now take the following steps:
Check with the athlete if he was recently in China, Mexico or Guatemala and whether he consumed meat there.
If so, check whether the test results are consistent with meat contamination.
If it all checks out, the process ends there. If not, they charge the athlete with an anti-doping rule violation and the athlete can explain himself in front of a tribunal.
In truth, this is more-or-less what was already happening. Anti-doping authorities decided pretty quickly after improved detection methods led to a flood of clenbuterol positives from Mexico and China that prosecuting athletes for eating meat didn't make any sense. On the other hand, they didn't want to give athletes carte blanche to take clenbuterol, so they tried a few different ways of dealing with it that essentially boiled down to: if the test result is very likely due to meat contamination, we'll drop the case. WADA has finally formalized their approach by publicly releasing these guidelines on how to deal with clenbuterol cases and by changing the anti-doping code.
Clenbuterol excretion takes a while afaik. This publication got its hands on an unpublished Mexican study commissioned by WADA which showed that clenbuterol was detectable in urine for about a week after eating contaminated meat, with peak concentration 23 hours after ingestion. Therapeutic or higher doses of clenbuterol would likely be detectable for even longer than that.