Some of it depends on the time frame, hence my question about how many years ago was the usage. In some cases, an investigator will need to seek people who have knowledge of the behavior. The obvious choice would be to interview folks who participated in the activity. Bear in mind, at no time are we gonna say “so, you and Bob smoked marijuana daily for three years, most recently on 1/1/23. Tell me about that”
We simply ask a question along the lines of “Has Bob used any illegal substances, including misusing prescription drugs” or “tell me about Sally’s use of drugs” Wording depends on the situation and the investigator. It’s best if people don’t lie, because then we may be required to keep interviewing people to find someone with knowledge of the activity. Plus lying when the report states that this person had first hand knowledge of the activity might make one wonder are they also lying about the person’s trustworthiness, integrity and honesty.