The first and most important way to find out if someone is using drugs is when your instinct tells you that this person has changed, or they no longer behave the way they used to a few weeks or a couple of months ago. The person is showing up late for appointments, cancelling family gatherings. You have caught them in more than one lie even when they blatantly tell you the lie and believe it. Things are beginning to go missing. You're suddenly losing money mysteriously. All the above and others are signs of drug use. These are not single events but reoccurring issues. The step to take next is to insist on a Urine Analysis, these can be purchased at local pharmacies, tell the person your concerns and let the UA test determine and clear away your doubts.
If the person is using regularly they may become very defensive and begin to accuse you of distrust and try to make you feel guilty. Just insist they do the test, be present for the test or have someone of same sex be present. If it is negative, then you will know something else is going on. If it’s positive, you can now deal with their drug addiction.