I can tell you about my daughter who quit. She is only one person and did not really do rehab or anything, but she wanted to quit and we immediately noticed big differences in her thinking and behavior and attitude toward her future. Remember that all people quit differently, but I think the behavior afterward is pretty similar...things vastly improve. It took me over a year though to believe she wouldn't relapse because she was doing some pretty heavy stuff.
The first thing that stood out was that she no longer hung around with drug users at all. She had moved to live in the next state with her religious, very judgmental brother, because otherrwise she'd have been homeless. He was not a really supportive person and thought she was terrible for using drugs, but he gave her his basement which was a roof and she wanted that roof enough to listen to his rules. She stopped fighting society. She followed his rules. She got a job. She had no car so she walked back and forth. She got promoted. She did not have any friends for a long time because she did not want to be around drug users or party people anymore due to wanting to stay straight and she was shy. Until she met her boyfriend, who she is still with eleven years later, she had nobody. She was lonely, but she preferred loneliness to the way her life had been on drugs. She stopped having moodswings that had once given her a false diagnosis of bipolar. She thought about her future and took out a loan to go to a two year college, got a degree, and life continues to be good for her and her SO and their new baby (they waited ten years to have a baby, who is being well taken care of by them). She does not get into trouble with the law, which she did while using drugs. She does not steal, which she did while using drugs. I do not doubt the truth of what she tells me, which was the exact opposite of what she did when she used drugs. She lied all the time then. She is just not the same at all. I don't doubt. I know.
One other thing that may matter if your son used speed, like my daughter did. She was stick skinny back then and it scared me. That is not her normal physique. She is more prone to have to fight chubbiness. Her skinniness and hollow eyes made me think she was going to end up in prison or die. She did neither. She quit. She quit for one reason only: She wanted to quit. She was "tired of the sh****** life of drugs." Drug dealers were after her, threatening her life because she owed them money. It was a real blessing when we threw her out and her brother, in another state, relunctantly and sternly allowed her to move in with him, even though he was far from nice about it. That allowed her to quit without being dogged by her former "friends." However, if she had wanted to use, there are plenty of drug users in ALL states. She just didn't want to.
It took her several tries to quit until she moved as the intense peer pressure was always there. And she was known as a bad kid so nobody nice wanted anything to do with her. Her fresh start helped her a lot.
When she first said she quit, we thought, "Yeah...right."
It took SEEING the many changes to believe it and then we still crossed our fingers.
She even stopped smoking cigarettes, which is a big relief to me because I do consider cigarettes serious. Now she has a "No Smoking" sign in the house she owns with her SO. She is into organics and does not like to take ANY medication. It's like a different person.
She drinks twice or three times a year. She does not try to get drunk and she and SO live a quiet life with their child.
These are ways I could tell my daughter was serious. Many addicts go to rehab. My daughter wouldn't and was not forced to by the law. Therefore, she didn't. However, I would think it is much the same no matter how you quit drug use. I think the biggest indicators are who your friends are, deciding to grow up and getting a job, and not asking us for things, especially money.
My daughter started smoking pot at 12. By 19 she had used everything, including psychodelics, meth, and a few tries with heroin. If you think that you try heroin once and you are automatically addicted, it's not true. She did and was not. Uppers were her drugs of choice, especially meth. Her complexion is still bad maybe because of using it. But she looks healthy and is chubby again. I am so happy to see her chubby and looking bright eyed and happy.
She is one of my heroes.