The Substance Use Motivation Ruler is an excellent tool derived from motivational interviewing. Ask your client to pinpoint, on a scale of 1 to 10, how motivated they are to end their drug use. In this case, 1 means "not at all motivated" and 10 means "completely motivated".
In the spirit of motivational interviewing, you want to use this worksheet to build congruence, and focus on what motivates the client...