When you are buying clones there is no way to know for sure if the clone the person is selling you is really the strain they claim. Unless you are dealing with someone that is deperate for money people are usually honest about what strain they are selling. It's not often easy to find but if possible you should buy a clone that was taken from a plant that was grown from seed. The growth, vigor and overall vitality of a clone that was taken from a plant grown from seed is much better than a clone that was taken from another clone. If someone says that the clones / plants they are selling came from a marijuana plant grown from seed, ask them if you can see it. If not, ask them if they will take a close up picture of the stalk. Looking at the stalk will enable you to confirm the plant was grown from seed. If they won't do either, there's a good chance they are lying.
How to Confirm A Plant was Grown From Seed
There are two characteristics that all marijuana plants grown from seed have that a plant grown from a clone / cutting will not have. The first thing to look for is to look at the branching at the base of the stalk. If the branch pattern is in pairs of two's (one branch growing at the same level as another branch but on the direct opposite side of the stalk) at the base of the stalk then it's almost definately a marijuana plant grown from seed. A cannabis plant grown from a clone / clipping will have staggered branching at the base of the stalk.
The other thing to look for is concentric rings (indentations) around the base of the stalk. This is something only a marijuana plant grown from seed will have.
How to Buy Clones / Marijuana Plants Checklist
You can't be certain the clone / plant you are buying is the strain the person says it is, but you can at least keep yourself from buying a clone with bugs or a disease. If you buy a clone with a disease or bugs you might introduce something to your garden that will take a lot of trouble and time to get rid of, such as spider mites or powdery mildew (often refered to as PM). Here's a thorough checklist to insure that you are getting a good healthy marijuana clone or plant. Please don't e-mail me asking where to buy clones, I do not have any personal experience with any of the places that sell clones so I can't recommend a place.
Clearing up a common misconception...
Many people seem to think that if they buy a clone or seed that is from an excellent strain of weed that won a cannabis cup that the plant they grow with this clone of a cannabis cup winning strain will end up being of the same quality just because of it's award winning genetics. Here's the truth, an amature grower can take a seed or clone of a strain like Trainwreck and the plant he grows may end up being crap with no more than 6% THC even though the strain has been tested at somewhere between 18%-22%. An experienced grower can take a seed from a bag of crappy import and the plant he grows from that seed will be as good as most of the MMJ you see at dispensaries.