The company I work for can determine gender based on first name with 80 percent accuracy. I never bothered asking how but most names are gender-specific. As mentioned on other comments on this page there are significant difference between the written expression of men and women although that gap may be narrowed when you are limited to 140 characters.
yes, I thought about it. but how many names can you save/check like that? won't it be a tedious process? Also, what about names from Europe, muslim names etc?
You build a database and program to match the names to a list. The database would contain only a few thousand records which would be easy to manage and speedy. Very simple process for a programmer. None of this work should be done manually.
For names you don't know you keep using other means to dig further. But no matter what tools I would use, matching names to gender would be highest weighted procedure and the first thing I'd try. Of course, there would have to be second and third things to try too.
Remember, these guys didn't say they were 100%. Who even knows what their definition of "pretty accurate" is.
Look for He/She in messages directed @User.