First off, if you know that they have a mental or educational disorder, try to exclude that from typing. Common mistakes include people with Aspergers or OCD typed as STJs and people with ADHD or ADD as SPs.
Then, try to learn about how different dominant functions work. Isolating the dominant function first will significantly narrow down possibilities, and it will also be easier to know what to look for. It's usually easier to do this with friends that you spend time out of work/school with, but if you aren't good friends with them, try to observe them with other colleagues, friends, or family. Remember, if their dominant is an introverted function, then they are introverted and will likely enjoy keeping to themselves and having alone time, and vise versa.
Once you have found their dominant function, try looking for their auxiliary, or what they use to back up their dominant. Try to learn about the key differences between the two types with the same dominant.
Another tip is to look out for their tertiary function. From an outside view, their tertiary function might seem like the most prominent thing about them, but also try to look deeper beyond behavior and into motivations. For example, I look like an ISTJ because I am very organized and structured from an outside perspective and I am very serious and no-nonsense, but everything that I do is guided by my personal feelings and beliefs, and I tend to snap when something personally offends or upsets me. In contrast, my ESTJ sister loves to tinker and toy with things and create science experiments in the kitchen and bathroom. However, her love of control and natural leadership abilities, as well as a concrete look at life set her apart from an Ne-dom.
Just a few things to consider.