how do you find out if police have evidence?

The attorney is typically the person who will access to the State's file. They're not going to release their evidence to a non-lawyer, for a variety of reasons. Unfortunately, you won't be able-on your own-to determine whether he's "innocent," or not, if and until an attorney reviews their file. Usually, you don't see an arrest for DUI and tampering. DUI is a class c offense, which would apply in the case of a 19 year-old stopped for suspicion of DWI (DUI is the offense applicable to underage drivers - the burden of proof required to convict a person of DUI is not the same as that which is required to convict an adult of DWI). If the officer was able to articulate "any detectable amount" of alcohol, then your son could in fact be guilty of DUI. Tampering, in these situations, usually implies a person destroyed potential evidence. If it was a drug case, and, for example, he swallowed whatever the substance was, that would considered tampering. And there is no such thing as an inconclusive breath test. It's either above the limit; or not. Again, the "limit," or 0.08 g/ml, does not apply in a DUI case.  Mostly, I've offered a bunch of educated guesses, given the limited facts provided. An attorney would very likely be able to answer more intelligently with additional information.