Step One: First join twitter.
Step Two: Figure out a way to work around the fact that your child has blocked you on twitter. I did it by using an alternative account that he doesn't know about. I don't follow him on this account, but just use it to check up on his timeline now and again.
Step Three: Mention to your child that there are some questionable tweets in his timeline and that being on-line doesn't give him leeway to be a jackass. Correct his on-line behavior (but don't do it on-line of course).
Step Four: Eventually you will then see a tweet similar to this one:
7/2/13, 8:49 PM
Mom, stop looking at my Twitter account. I'm 19 years old. Not to mention I blocked you. Learn to take a hint, sheesh!
Step Five: Continue to check-up on their social media activity even though they think they don't need supervision. Teenagers, even the smartest ones (especially aspergeans, considering most have social interactive issues to begin with), can post some really damaging things that could come back to haunt them in later years.
It also doesn't hurt to make sure they aren't interacting inappropriately with some pervert too. Teens and adolescents think they would be able to identify a sexual predator, but they can't. Sadly predators are very good at what they do.
Step Six: Sleep well at night.
On-line Trolls; IRL Trolls; Cyberbullying and the Ability to Block