Virginia's Driving While Intoxicated/Under the Influence law has gone through eras of progression and as it stands today includes much more than just alcohol. Due to the rise of drugged driving people can be charged with DUI without so much as a drop of alcohol to drink. Many have been arrested for the admittance of smoking marijuana, and the finding of THC in their blood.
How do they prosecute DUI without a breath test? People charged with DUI alleging an influence of drugs obviously won't involve a breath test, so the officer who arrests a person usually asks them to submit to a blood test. That blood is then sent off to the Department of Forensic Science, a state run lab, to test it for a variety of different drugs. Depending on what drugs are registered in the blood the State then tries to determine if they can make a case that it is the influence of that drug (or combination of drugs) that rose of a level of influencing a person's driving behavior.
How does an officer know they are under the influence of anything? The officer looks for clues such as violations of the traffic laws, the person's disposition, smell of marijuana, bloodshot or dilated eyes, speech, mannerisms, their appearance and any statements of drug use to determine if the accused is under the influence of any drug or drugs.
The National Highway and Traffic Safety Administration (NHTSA) has recently come out with a new survey stating that drugged driving is on the rise, so police officers are now keenly aware to look out of any influence of drugs when stopping someone even for just a simple traffic violation. A link to the study can be found here: http://www.nhtsa.gov/About+NHTSA/Press+Releases/2015/nhtsa-releases-2-impaired-driving-studies-02-2015)
Drive While Under the Influence is a very serious charge and when dealing with drugs (even legally prescribed drugs!) the laws and defenses are highly complex so you need an experienced attorney to look into these cases if you are accused of them.