Yes, the police commonly utilize three methods to determine whether someone is driving under the influence of marijuana. The first and most simple method is to look for evidence of marijuana use. For example, officers may see a joint or blunt in plain view inside the car or smell marijuana smoke coming from the car.
Second, a Drug Recognition Expert (“DRE”) will evaluate the driver. A DRE is an officer whose received additional training on how to detect whether someone is under the influence of drugs. This training does not grant them super powers but rather teaches them to look for common indications of marijuana use like blood shot eyes, droopy eye lids, or slowed speech.
Third, officers may seek to have the person driving undergo a blood test. Depending on the results of the blood test, the prosecution will then use the test to prove someone was high at the time of driving. Blood tests for marijuana look for byproducts of marijuana use called metabolites. Using blood test results, however, can be tricky because the two kinds of metabolites that may be present, active and inactive, don’t necessarily mean that someone was impaired at the time of driving. For example, regular users of marijuana can have metabolites in their blood even when not impaired. Also, different users have different levels of tolerance so while one level of metabolites can mean impairment for one particular person, it doesn’t necessarily mean impairment for another.
All that said, the prosecution will seek to use any means at its disposal to prove its case. For example, if there was a car accident or the suspect gave incriminating statements, the prosecution will use that evidence at trial. The prosecution will also attempt to persuade the jury by pointing out that while the law does not forbid using marijuana, it does forbid using marijuana and driving. If you’ve been arrested for  DUI involving marijuana, the best thing you can do is to hire an attorney to review your case and use expert witnesses to evaluate the evidence against you.