Many people use forensics linguistics every day and don't even realize it. For instance, I'm from the Southeast, and even the best trained actors can rarely hide it from me if they grew up in the South. I can hear it in their voices. If they don't bother to hide their accents, I can usually get the state they're from. I suppose Professor Henry Higgins was, strictly speaking, a dialectologist, but there are forensic linguistics at work there, too.
Also, once one becomes accustomed to reading a certain person's written work, he or she can usually recognize it again. It's also fairly easy to figure out whether someone is a native speaker of a language, and if the language is spoken in more than one country, where the writer is from. For instance, I can almost always spot whether a writer is a native English speaker. There are errors that English speakers make that foreign speakers do not make and vice versa. If they are native English speakers, I can also usually tell whether they are American, British, Canadian, etc. It's not easily explained, but when you've done it enough, you just know the difference.