To prepare for an intervention, Dr. Phil recommends everyone bring a list of factual data over a period of time that spells out without a doubt that the person is using drugs or alcohol. Include the date, what happened, what was said and done and how it made you feel. Make sure there is no speculation, only facts.
For example: "In August, 2001, I overheard you ordering drugs over the phone. I confronted you, but you denied it and told me I was crazy. I cried myself to sleep that night. I was angry about your accusations and hurt that you wouldn't talk to me."
You can look for specific data in these areas:
Changes in pattern of use: denies use, hides supply, uses alone, increased use, can't stop using, increased tolerance, etc.
Behavior when using: becomes happy/animated/angry, becomes violent, increased arguments/fights, becomes withdrawn/silent, up all night/sleeps all day, etc.
How use is affecting: job, finances, relationships, health, responsibilities, safety, family, etc.
Here is a worksheet to help you get started.