Is their THC use disruptive to normal life?
Is their THC use a coping mechanism that's been adopted due to certain life circumstances?
Is their THC use an impediment to their social life?
Is their THC use something that started in their teen years that had continued into adulthood?
If the answer is yes, then addiction is highly probable.
Even so, gentle ways of simply reaching out, connecting, hanging out, and building a stronger social bond can help you determine if intervention is necessary (and these acts might even quell the use).
Still, there are differences between addiction and having a bad habit or being a functioning addict.
Not many people can hide an alcohol addiction well, but there are many people who self medicate with a variety drugs that you'd never have guessed. There is a reason the black market in drugs is so profitable and it's not because of a few loser addicts that ruin their own and others lives, shit loads of people that you'd never have suspected are regularly taking this or that drug.
If someone smokes weed as a coping mechanism, but they can function normally, hold down a job and a relationship and everything else, then power to them.
The problem is that a functional alcoholic is still completely disassociated from reality when heavily drunk, you just can't recognize it from their behavior.