What happens if a cop suspects you are high?

Always remember: you can beat the rap; you can't beat the ride. Translation: if the cop suspects you of just about anything, it's likely they can justify an arrest and take you to jail.  To answer your question directly: there is a test called a DRE that cops use nowadays to try to determine whether a person is high. I'm not saying it's a reliable test, or in any way a reliable arbiter of recent drug use, but it's the current test being used by them to substantiate such claims. Mostly, it's used in DWI cases, but there is certainly the potential a cop could call in a DRE on a PI case.  Also, they often use one of the DWI field sobriety tests as a tool for justifying probable cause to arrest for PI. If a cop asks you to stand still, and follow the pen/pen light/finger with your head only, he is using that test. Also, if you smell like weed, the case law will allow them to stop you and investigate. If they smell it in your car they can also stop and investigate. Red eyes can be a factor used in a probable cause analysis, as well. If they observe those things I just mentioned, and you deny being high, they can still ask you for a urine or blood sample. You can still refuse. And they can then, if it's that important to them, draft up a warrant application and ask a magistrate to sign a warrant for your blood. So: if you smoke - keep an extra clean shirt handy, chew gum, and deny, deny, deny.