It is confusing and there is no way to make sure, unless you have some background knowledge about the subject. The only way to avoid confusion is to reword the sentence in a clear way, like twice a week, or every two weeks. By the way, Tim, Biannual means twice a year. The one meaning every two years is "biennial". Again many people get confused in this case! I wonder why they can't just say twice a year or or every other year/ every two years to avoid all the hassle!

Morpheus is correct about 'biannual'. Sorry for addnig even more confusion to this topic. : /

I agree with fofa. And you are right, it is very confusing. We have the same problem with 'bimonthly'. But not with 'biannual'. 'Biannual' always means 'every other year'. If you are unable to understand 'biweekly' from its context, then you should probably assume it means 'every other week', because this is its usual meaning.

biweekly is often avoided as twice a week , there is another word to mean twice a week; semiweekly also every two weeks means fortnightly