Suspect a family member of drug use, don't know what to do (asked in another category too)?

My stepdaughter has decided to separate from her husband and they have two young kids together. They rent a house from us just up the street. Last night she came over to talk and her appearance made me suspect that she was on some drug. She looked exhausted, had just showered, and her pupils were tiny pin pricks. She has recently begun hanging around a crowd that is known for partying and has proclaimed that she no longer is in love with her husband and wants to separate. Her husband is beside himself and has mentioned that they were partying with this crowd and using drugs, that he stopped but he suspects she has not stopped. I don't know if he is saying this to gain sympathy and how much of it is true. Up until a few weeks ago my stepson was living in their house and he claims that he never saw anyone take drugs other than ibprofen, but I don't know if he's trying to protect her. I am considering buying a drug test from the pharmacy and asking her to take it, but beyond that I don't know what to do. If her new friends are lousy influence, perhaps her decision to break up her family has more to do with them and drugs and if we could get her away from them she might change her attitude. How do we go about an intervention? If she's taking drugs, she is endangering her children AND she could become addicted....