There is a difference between alcohol and drugs, yes. In detection it's relatively simple to use a screening device to determine whether the person has alcohol in their body, but for drugs there is no such testing roadside; there is not an instrument like that. It means you have to do the standard field sobriety test at the roadside in order to detect, and you're not actually saying whether there's a drug or alcohol or a combination of both; it's whether the person has difficulty doing divided attention tasks. That suggests to you that they would be having difficulty in safely operating a motor vehicle.
Only a certain small number of those tests, three roadside tests, are done. If a person fails those, they then go back to the police station, where they would undergo a DRE evaluation, which includes divided attention tasks but also a number of other clinical indicators.
It would probably be fine to videotape the standard field sobriety test of having the person go through the walk and turn, the one-leg stand, etc. However, the clinical indicators are things like taking blood pressure, pulse, and body temperature. I'm not exactly sure what you expect to see by videotaping that; it really won't show anything. There would be limited value in doing a videotaping of a DRE evaluation, because other people are only going to be able to see—If you don't know what you're looking for and are not trained—'m not sure of the value of having it videotaped.