Just as important as knowing if you can represent yourself in legal matters is when you shouldn't.
First, and most important, if you don't have the time, inclination, or if you aren't studious then you shouldn't attempt to handle your own legal matters. When I once did a calculation as to how much time I spent preparing for trial as opposed to actually being in trial it appeared to be about 25 to 1. In other words, for every hour I was in court on a matter I spent about 25 hours outside of court working on the case in one way or another.
A lack of a formal legal education would mean you could expect to add another 10 hours, at least, onto that time so you can reasonably expect to spend about 35 hours outside of court for every hour you spend in court. This is not only due to unfamiliarity with the system but also because some things lawyers learn in law school and in general practice will require you to learn them for your specific case.
Second, only a fool would choose to represent themselves in a criminal case where jail time is a possibility. It's a different matter if you can't afford a lawyer (as opposed to just not wanting to pay) and the court refuses to appoint one. Even lawyers don't represent themselves when they are facing criminal charges so that should tell you how important it is to have neutral and detached counsel who can look at the evidence and the facts, as well as any plea bargain offers, and give you their honest opinion as to the case.
Third, a person should never handle their own real estate transactions even if they are a part of a probate, a divorce, or a real estate matter in and of itself. Real estate is extremely confusing and if you don't do a lot, you shouldn't do any. In addition, any mistakes that are made may not be found for a very long time and, once discovered, it can cost a substantial amount of money to correct a mistake, far, far more than it would to have hire someone to do the matter correctly in the first place.
Divorces are iffy. Many people choose to represent themselves and if there is no significant amount of property and no children, and the parties are in agreement then it is pretty safe to handle the matters yourself. However, the presence of any of these factors should make a person carefully consider before representing themselves.
Finally, if the other side on a case is represented by an attorney a party would be wise to give serious consideration to hiring a lawyer themselves. As the website has set forth before, it's not necessarily that an attorney is smarter than a pro se litigant, it's just that they have more experience and know how to handle any unexpected matters that pop up and, in addition, a lawyer can spot matters a pro se litigant will not.
The whole purpose of LessonsInLaw.com is to help people represent themselves but a part of this is knowing when an attorney is needed.