There is an events calendar here at Taiwanderful. Friday's Taipei Times has listings of events and is an excellent source of information. The other English-language newspapers also have listings.

I also suggest picking up flyers and looking out for posters/advertisements around town. Although they are usually in Chinese they will often have some English written on them. You can also check the websites of the various venues, although the English versions are not always up to date. 

Although there is no definitive source of information in English if you glean information from various places you can find out about most things.